Add tests for external network extension

Add tests for:
 - Create a network from an administrative user specifying the
   external network extension attribute;
 - Update a network from an administrative user specifying the
   external network extension attribute;
 - List networks from a normal user and confirm the external
   network extension attribute is returned for those networks
   that were created as external;
 - Show an external network from a normal user and confirm the
   external network extension attribute is returned.

partially implement bp: improve-neutron-test-coverage

Change-Id: I3e4978b95d0f0194a698f3b7929f90c23cb8345e
Merge "Add tests for external network extension"