Adding support for RedHat Subscription Manager repository subscriptions ...

...with rhsm_repo

Adds new type rhsm_repo.  Supports adding and removing. Syntax:

rhsm_repo { 'rhel-7-server-optional-rpms': ensure => present }
rhsm_repo { 'rhel-7-server-optional-rpms': ensure => absent }

Makes use of : /var/lib/rhsm/cache/written_overrides.json to enumerate existing regstered repos.  Not sure if this is the best way to do this, but it seemed less expensive than `subscription-manager repos-list`