Description of problem: Post upgrade check(upgrade from 6.7 to 6.8) failed with  error  "undefined method `to_sym' for nil:NilClass"

Version-Release number of selected component (if applicable):
6.8

How reproducible:
2/2

Steps to Reproduce:
1. Prepare the environment to perform the satellite-upgrade from 6.7 to 6.8 
2. Configure the all required repository to perform the upgrade from 6.7 to 6.8.
3. Apply the workaround mentioned on BZ#1846593
   a: unlock the fm-packages.
      foreman-maintain packages unlock -y
   b: Install puppet-agent 
      yum install -y puppet-agent
      .....
      Updated:
      puppet-agent.x86_64 0:6.14.0-2.el7sat
   c: re-install puppet-agent-oath 
      yum reinstall -y puppet-agent-oauth 
      ....
      Installed:
      puppet-agent-oauth.noarch 0:0.5.1-3.el7sat
   d: lock the packages
      foreman-maintain packages lock -y
4: Run foreman-maintain upgrade check and it completes with some warning.
5: Run foreman upgrade using below command

foreman-maintain upgrade run --whitelist="disk-performance, repositories-validate, repositories-setup" --target-version 6.8 -y
 Checking for new version of satellite-maintain...

 Running Migration scripts to Satellite 6.8
 ================================================================================
 Setup repositories:                                                   [SKIPPED]
 --------------------------------------------------------------------------------
 Unlock packages:                                                      [OK]
 --------------------------------------------------------------------------------
 Update package(s) :                                                   [OK]
 --------------------------------------------------------------------------------
 Procedures::Installer::Upgrade:                                       [OK]
 --------------------------------------------------------------------------------

Running Procedures after migrating to Satellite 6.8
 ================================================================================
....
....
Running Checks after upgrading to Satellite 6.8
 ================================================================================
 Check number of fact names in database:                               [OK]
 --------------------------------------------------------------------------------
 Check for verifying syntax for ISP DHCP configurations:               [OK]
 --------------------------------------------------------------------------------
 Check whether all services are running:                               [OK]
 --------------------------------------------------------------------------------
 Check whether all services are running using the ping call:           [FAIL]
 Couldn't connect to the server: undefined method `to_sym' for nil:NilClass
 --------------------------------------------------------------------------------

Actual results:
Post-upgrade check failed with undefined method error.

Expected results:
Post-upgrade checks should be completed successfully.

Additional info: