engine : Bll query to get GeoRep Sessions for volume

This patch provides bll query for obtaining gluster
geo rep session list for a given volume id

Bug-Url: https://bugzilla.redhat.com/1138116
Change-Id: Ie4c6c372d3680dc9d13cf31ade3b640ee3c86819
Signed-off-by: Anmol Babu <anbabu@redhat.com>
Signed-off-by: Sahina Bose <sabose@redhat.com>