1035440: Don't rewrite redhat.repo unless it has changed
refresh ent_dir after adding/deleting certs
Merge pull request #949 from candlepin/ckozak/avoid_writing_repo

Ckozak/avoid writing repo
Support getting release versions via API call

Extracted release version providers for CDN and API implementations.
ReleaseBackend chooses a provider based on whether the server
supports the 'available_releases' resource.
Merge pull request #954 from candlepin/mstead/sat6-release-version-api

Support getting release versions via API call
Plugin to prep a docker build host for entitlements