core: delete connections on block domain removal

When removing a block storage domain, the relevant connections must
be removed from the DB before removing the corresponding luns
(as we can't retrieve connections by VG without the luns).

Change-Id: If1be645779e5ad28e9c2672246e6e68d5aecb00e
Bug-Url: https://bugzilla.redhat.com/1011569
Signed-off-by: Daniel Erez <derez@redhat.com>