Description of problem:
Activation key page doesn't load when an Activation key got created without the candlepin pool id

Version-Release number of selected component (if applicable):
Satellite 6.2.14

How reproducible:
- Always by manipulating database table.
- Not sure yet on how this happened in production, but so far we saw this with one customer.

Steps to Reproduce:
1. We are not exactly sure on how this happened at the customer environment.  But they have been restarting katello-service often and have multiple scripts run by multiple engineers to perform satellite day to day activities. But I can reproduce this with table updates as shown below.

# sudo su - postgres
# psql foreman

foreman=# select * from katello_activation_keys where name = 'testincorrectak';
 id |      name       | description | organization_id | environment_id |         cre
ated_at         |         updated_at         | user_id | max_hosts | content_view_id
 |              cp_id               | release_version | unlimited_hosts | auto_attac
h 
----+-----------------+-------------+-----------------+----------------+------------
----------------+----------------------------+---------+-----------+----------------
-+----------------------------------+-----------------+-----------------+-----------
--
  3 | testincorrectak |             |               1 |              2 | 2018-04-13 
20:23:42.340106 | 2018-04-13 20:23:43.224347 |       3 |           |               3
 | ff80808162ac47270162c0ad91200115 |                 | t               | t
(1 row)

foreman=# update katello_activation_keys set cp_id=NULL where name = 'testincorrectak';
UPDATE 1
foreman=# select * from katello_activation_keys where name = 'testincorrectak'; id |      name       | description | organization_id | environment_id |         cre
ated_at         |         updated_at         | user_id | max_hosts | content_view_id
 | cp_id | release_version | unlimited_hosts | auto_attach 
----+-----------------+-------------+-----------------+----------------+------------
----------------+----------------------------+---------+-----------+----------------
-+-------+-----------------+-----------------+-------------
  3 | testincorrectak |             |               1 |              2 | 2018-04-13 
20:23:42.340106 | 2018-04-13 20:23:43.224347 |       3 |           |               3
 |       |                 | t               | t
(1 row)

Actual results:
After the above table update, go to Satellite UI -> Activation Keys. The page will load forever and will not finish.
Error in production.log:
2018-04-13 16:41:48 [app] [E] RestClient::ResourceNotFound: Katello::Resources::Candlepin::CandlepinResource: 404 Resource Not Found {"displayMessage":"Runtime Error RESTEASY001185: Could not find resource for relative : /activation_keys//content_overrides of full path: https://localhost:8443/candlepin/activation_keys//content_overrides at org.jboss.resteasy.core.registry.PathParamSegment.matchPattern:209","requestUuid":"269c5713-8946-4344-8cc3-76d257195b39"} (GET /candlepin/activation_keys//content_overrides)
 | Body: {"displayMessage":"Runtime Error RESTEASY001185: Could not find resource for relative : /activation_keys//content_overrides of full path: https://localhost:8443/candlepin/activation_keys//content_overrides at org.jboss.resteasy.core.registry.PathParamSegment.matchPattern:209","requestUuid":"269c5713-8946-4344-8cc3-76d257195b39"}
 | /opt/theforeman/tfm/root/usr/share/gems/gems/rest-client-1.6.7/lib/restclient/abstract_response.rb:48:in `return!'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/rest-client-1.6.7/lib/restclient/request.rb:230:in `process_result'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/rbovirt-0.0.38.1/lib/restclient_ext/request.rb:50:in `block in transmit'
 | /opt/rh/rh-ruby22/root/usr/share/ruby/net/http.rb:853:in `start'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/rbovirt-0.0.38.1/lib/restclient_ext/request.rb:44:in `transmit'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/rest-client-1.6.7/lib/restclient/request.rb:64:in `execute'
 | /opt/theforeman/tfm/root/usr/share/gems/gems/rest-client-1.6.7/lib/restclient/request.rb:33:in `execute'

Expected results:
Activation key page works fine if an activation key exists without candlepin pool id for whatever reason.
There can be an error or warning saying that a specific activation key needs to be removed.

Additional info:
workaround:
foreman=# delete from katello_activation_keys where name = 'testincorrectak';
DELETE 1