engine: Gluster hook content query

Fetch and return the gluster hook content either
from db or from vdsm based on the parameters.

If server id parameter is null it will fetch
the hook content from db otherwise from the server vdsm.

Signed-off-by: Selvasundaram <sesubram@redhat.com>
Change-Id: Ie24d1132fc214661cd2c2e3071f641812d19cd4f
Signed-off-by: Sahina Bose <sabose@redhat.com>