yum-validator: improve RHSM repo detection

This changes `repo_is_rhsm` so that the RHSM `RepoActionInvoker` method
`is_managed` is used instead of comparing the filename. This will work
even on systems where `manage_repos = 0` is set in `rhsm.conf` but a
valid subscription exists and repos from entitlements are provided from
a file other than `redhat.repo`

Also, `is_managed` is used to prevent repo overrides from being set on
systems that have that feature disabled.