Further fixes concerning drbd port release

Commit 3b3b1bca does not entirely fix the bug introduced in commit
f396ad8c. It fixes consistency of config data in permanent storage, but
does not ensure consistency in data held in runtime memory of masterd.

The bug of duplicate ports is still triggered when LUInstanceRemove()
invokes _RemoveDisks() and this returns False (in case
call_blockdev_remove RPC fails). The drbd ports get returned in the
pool, but execution is aborted and RemoveInstance() is never invoked.

Due to the fact that port handling is not done with
TemporaryReservationManager, ensure that ports are released, only if
disk related config data is deleted.

In _RemoveDisks() release ports only if all RPCs succeed.

Extend _RemoveDisks() to include ignore_failures argument passed by
_RemoveInstance() to handle the ports appropriately.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Michael Hanselmann <hansmi@google.com>