engine: Refactoring GlusterManager

Renamed GlusterManager to GlusterSyncJob.
Added a base class GlusterJob that all gluster job
classes can extend from.

Created a class GlusterJobsManager that's responsible
for scheduling gluster jobs.

Change-Id: I61d851928f18da22b352b7be9d6ccaf4382e7930
Signed-off-by: Sahina Bose <sabose@redhat.com>