Fixes #4758: Adds client authentication to enabled repos call.

After moving the client authentication solely to the Candlepin proxies
controller, the enabled repos call from katello agent could no longer
complete successfully. This slightly re-factor the client cert to be
generic to any client providing an SSL cert and includes this authentication
method for the enabled repos call in the V1 systems controller.
Merge pull request #3859 from ehelms/fixes-4758

Fixes #4758: Adds client authentication to enabled repos call.