Description of problem:
-----------------------
When many Host Groups  have been associated to a Provisioning Templates by using "Add combination", the Provisioning Template is impossible to save and throws the following Error:
  ---
  Internal Server Error
  The server encountered an internal error or misconfiguration and was unable to complete your request.
  Please contact the server administrator at [no address given] to inform them of the time this error occurred, and the actions you performed just before this error.
  More information about this error may be available in the server error log.
  ---


Version-Release number of selected component (if applicable):
-------------------------------------------------------------
6.3.0


How reproducible:
-----------------
100%


Steps to Reproduce:
-------------------
1. Create a New Operating System, so we do not need to alter the working ones.
   Select  x86_64 as architecture

2. Create a test provisioning template, by example you may clone "Satellite Kickstart Default" to "0_Test". 
   Associate this test template to the newly created test Operating System, as well as the proper Organization and Location

3. Edit the Operating System to make sure to change the Provisioning template from "Satellite Kickstart Default" to "O_Test"

3. From the shell on Satellite server
   3.1 Set the variables according to your environment
       -Organization
        # hammer organization list
        # org_id="1"
  
       -Operating system
        # hammer os list
        # os_id="9"

   3.2 Create 60 HostGroups
       # for i in $(seq 1 60); do hammer hostgroup create --name "HG_Test_$i" --architecture-id 1 --operatingsystem-id $os_id --organization-ids $org_id; done   

4. Add combinations
   From the WebUI, edit the provisioning template to add as much Host Groups as possible under "Association" (setting the Environment is not required) and click on Submit.
   
5. Reload the Provisioning Template and click on "Submit", it will throw the following error in the WebUI:
  ---
  Internal Server Error
  The server encountered an internal error or misconfiguration and was unable to complete your request.
  Please contact the server administrator at [no address given] to inform them of the time this error occurred, and the actions you performed just before this error.
  More information about this error may be available in the server error log.
  ---

Actual results:
---------------
A. From the WebUI the following is experienced:
  ---
  Internal Server Error
  The server encountered an internal error or misconfiguration and was unable to complete your request.
  Please contact the server administrator at [no address given] to inform them of the time this error occurred, and the actions you performed just before this error.
  More information about this error may be available in the server error log.
  ---

B. From /var/log/foreman/production.log, no error are seen but a entry for each combination can be noticed:
  ---
  [...]
  2018-04-09 17:01:30 8736f6c5 [app] [I]   Rendered 
  provisioning_templates/_combination.html.erb (9.0ms)
  2018-04-09 17:01:30 8736f6c5 [app] [I]   Rendered 
  provisioning_templates/_combination.html.erb (9.3ms)
  2018-04-09 17:01:30 8736f6c5 [app] [I]   Rendered 
  provisioning_templates/_combination.html.erb (8.5ms)
  2018-04-09 17:01:30 8736f6c5 [app] [I]   Rendered 
  provisioning_templates/_combination.html.erb (8.0ms)
  2018-04-09 17:01:31 8736f6c5 [app] [I]   Rendered 
  provisioning_templates/_combination.html.erb (8.4ms)
  2018-04-09 17:01:31 8736f6c5 [app] [I]   Rendered 
  provisioning_templates/_combination.html.erb (8.3ms)
  2018-04-09 17:01:31 8736f6c5 [app] [I]   Rendered 
  provisioning_templates/_combination.html.erb (8.0ms)
  [...]
  ---

C. From /var/log/httpd/error_log the following error is thrown:
   [...]
   [ 2018-04-09 23:28:12.2261 49366/7f42ad5ee700 Pool2/Implementation.cpp:1274 ]: [App 77156 stderr] [ 2018-04-09 23:28:12.2258 77232/0x0000000005aa0d70(Worker 1) utils.rb:74 ]: *** Exception 
   Rack::Multipart::MultipartPartLimitError in Rack application object (Too many open files - Maximum file multiparts in content reached) (process 77232, thread 0x0000000005aa0d70(Worker 1)):
   [ 2018-04-09 23:28:12.2261 49366/7f42ad5ee700 Pool2/Implementation.cpp:1274 ]: [App 77156 stderr]       from /opt/rh/rh-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/multipart/parser.rb:58:in `block in parse'
   [ 2018-04-09 23:28:12.2261 49366/7f42ad5ee700 Pool2/Implementation.cpp:1274 ]: [App 77156 stderr]       from /opt/rh/rh-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/multipart/parser.rb:56:in `loop'
   [ 2018-04-09 23:28:12.2261 49366/7f42ad5ee700 Pool2/Implementation.cpp:1274 ]: [App 77156 stderr]       from /opt/rh/rh-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/multipart/parser.rb:56:in `parse'
   [ 2018-04-09 23:28:12.2261 49366/7f42ad5ee700 Pool2/Implementation.cpp:1274 ]: [App 77156 stderr]       from /opt/rh/rh-ror42/root/usr/share/gems/gems/rack-1.6.2/lib/rack/multipart.rb:25:in `parse_multipart'
   [...]

  
Expected results:
-----------------
Do not have such en error, or set a limit of possible combination with a warning to the user.


Additional info:
----------------
The issue was not happening prior to Satellite 6.3.0 and the issue is now happening most likely because of a change in 
/usr/share/gems/gems/rack-1.6.4/lib/rack/multipart/parser.rb

In /usr/share/gems/gems/rack-1.6.4/lib/rack/multipart/parser.rb, we can see there is a new limit defined:
  "raise MultipartPartLimitError, 'Maximum file multiparts in content reached' if opened_files >= Utils.multipart_part_limit"