Validate delete_router before Midonet ops

* Move the neutron changes to the delete_router operation to before Midonet's
  operations. This gives the neutron validation a chance to abort the
  operation before Midonet has deleted its associated objects.

Fixes MN-1147
Signed-off-by: Joe Mills <joe@midokura.com>
Merge pull request #85 from JoeMido/MN-1147

Validate delete_router before Midonet ops