refs #5184 - fix couple of issues when deleting foreman hosts

This commit addresses a couple of issues that were observed with the
foreman host <-> katello content host integration, in the production
configuration.

1. Deleting the foreman host would result in a 404 to the user.
2. Unregistering a content host and visiting the foreman host would
   show an 410 error to the user.

The key issues that caused these were:
a. There were 2 bindings of the host extensions to the host.  This
   triggered the deletion of the content host twice.
b. The model relation for host and content_host did not have the
   proper 'inverse_of' definition, so it could lead to foreign key
   errors.
Merge pull request #4188 from bbuckingham/issue-5184

refs #5184 - fix couple of issues when deleting foreman hosts