Allow yum plugin to generate redhat.repo when unregistered.

Inside containers, we need to allow redhat.repo to be generated when we have
entitlement certificates, but no identity.
Don't encourage registration in yum plugin if we have ents but no identity.

In containers, we will have entitlements but no identity, we do not want to
encourage users to run register every time they use yum in this situation.

Also fixing a logic error, if you had no identity, and no ents, we were
prompting you to subscribe. (when we should be prompting you to register)
Disable CLI if we are running inside a container.

Leverage python-rhsm code to detect if we're running inside a container, if so
we disable the CLI completely.
Log that we're in container mode from yum plugin.
Merge pull request #963 from candlepin/dgoodwin/container-mode

Dgoodwin/container mode