engine: Change to geo-replication entities

Added a collection of GeoRepSessionDetail objects
to GeoRepSession and introduced volumeName as a
property
GeoRepSessionDetail is not a business entity on
its own. So changed the interface implemented

Change-Id: I5367f24dd677b8628d90459b4ba8115c665bfde7
Bug-Url: https://bugzilla.redhat.com/1125845
Signed-off-by: Sahina Bose <sabose@redhat.com>