BZ #1086344 - Enhance Cinder with GlusterFS support

https://bugzilla.redhat.com/show_bug.cgi?id=1086344

This is addition to the "Add Cinder service to HA hostgroup" BZ -
support for GlusterFS backend for cinder-volume running on the
controller nodes. The GlusterFS backend is now default.

Changed parameter default values - especially important is defaulting
gluster_servers to [] so that Foreman registers the parameter type as
array.