Fixes #7553: Update ConsumerIdentity location.

Subscription manager library moved the location of ConsumerIdentity
that katello-agent relies on. Instead of being at:

subscription_manager.certlib

The ConsumerIdentity class looks like it is located at:

subscription_manager.identity

This fix should work for all versions of subscription-manager, not just
the latest.
Merge pull request #12 from ehelms/fixes-7553

Fixes #7553: Update ConsumerIdentity location.