nova: enable RBD on RHEL 7
nova/rbd: use operatingsystemmajrelease fact & fix tests

* Fix the condition with () and use operatingsystemmajrelease to match
  the correct fact.
* Update unit tests to cover more situations of deployments.
Merge pull request #694 from enovance/enable-rbd-on-rhel

nova: enable RBD on RHEL 7