Add api tests for neutron router

Add tests for:
- Update router administrative state
- Add several interfaces (to different subnets) to a router
Add improvements for:
- When removing an interface, assert correct subnet id from the response
- When removing an interface, assert correct port id from the response

according to https://etherpad.openstack.org/p/icehouse-summit-qa-neutron

Change-Id: I538e8bed2f5c726ebd610d406702d4ef9de88723
Merge "Add api tests for neutron router"
Merge branch 'master' of https://github.com/openstack/tempest