Add rubygem-ovirt_provision_plugin package

Scratch builds:
http://koji.katello.org/koji/taskinfo?taskID=122698 (el6)
http://koji.katello.org/koji/taskinfo?taskID=122699 (el7)
http://koji.katello.org/koji/taskinfo?taskID=122702 (f19)

@bronhaim's the author of the plugin.

Will be built for nightlies only, due to rbovirt dependency.