Add extension "subnet-external-network"

Added extension "subnet-external-network" to:
* neutron-tempest-plugin-base
* neutron-tempest-plugin-openvswitch
* neutron-tempest-plugin-openvswitch-iptables_hybrid

The ``SubnetsSearchCriteriaTest.test_list_validation_filters`` test
case now filters the subnets by 'shared' and 'router:external' fields,
same as the network test case. It should behave the same with this
new extension.

Related-Bug: #2051831
Change-Id: I13144e3d722c06c918cd47860f378bf4bdaa0bf7
2 files changed