engine: Removed un needed calls to DB at VdsManager

The following patch is removing un needed queries to db, such
query was retrieving VdsGroup from db.
Reason: no reason, one time is enough.

Change-Id: I8f6b3c214d0b5606b6bff3261d9a0ea6ed36a124
Signed-off-by: Michael Kublin <mkublin@redhat.com>