Add tests to cover neutron-agents

This patch adds tests to cover next policies:

'get_l3-agents' [0]

Part of "Increase Neutron RBAC Coverage" initiative [1]

[0] https://developer.openstack.org/api-ref/network/v2/index.html#list-l3-agents-hosting-a-router
[1] https://storyboard.openstack.org/#!/story/2002641

Change-Id: I79310ce3b072976cff46335e233c219625c5b686
Story: 2002641
Task: 22312
1 file changed