vdsmapi: vdsmapi support for gluster vdsm verbs

Reads vdsmapi-gluster-schema.json from gluster directory if
vdsm-gluster is installed.

Change-Id: Id10b51a41d103afcf81cb76c460f3b9821238317
Signed-off-by: Aravinda VK <avishwan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13921
Reviewed-by: Timothy Asir <tjeyasin@redhat.com>
Tested-by: Timothy Asir <tjeyasin@redhat.com>
Reviewed-by: Adam Litke <agl@us.ibm.com>