delete_network should use neutron validation

* Neutron has some validation for deleting a network. This could prevent a
  network from being deleted. Therefore we should first let neutron
  validation bail out of the delete_network call before deleting
  the midonet side resources.
* This is achieved by simply executing the midonet side code after neutron.

Fixes MN-1338
Signed-off-by: Joe Mills <joe@midonet.com>