engine: Report domain monitoring for virt nodes only

Storage domain monitoring is required only for
virtualization nodes (not for gluster nodes)

Change-Id: I8731a81de756cbe0bb221f205c2a6d50c015b86b
Bug-Url: https://bugzilla.redhat.com/1105513
Signed-off-by: Sahina Bose <sabose@redhat.com>