Fixes #7553: Update ConsumerIdentity location.

[ehelms]

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.

[jlsherrill]

This would break compatibility with older versions. We'll have to do something like:

try:
  import new_module
except ImportError:
  import old_module

[jlsherrill]

Actually nevermind, it shouldn't break compatibility

[jlsherrill]

ACK, tested, worked well