RBAC tests for Tempest network agents_client

Implements RBAC tests for Tempest network agents_client, providing
coverage for the following policies:

  * update_agent
  * get_agent
  * create_dhcp-network
  * delete_dhcp-network
  * get_dhcp-networks
  * create_l3-router
  * delete_l3-router
  * get_l3-routers

This covers all the current endpoints in Tempest's network
agents_client, except for delete_agent (which is too destructive
to test in the gate) and list_agents, which doesn't do policy
enforcement.

Change-Id: Id1f64669860d9527f445398c59593ec16d7c6805
Closes-Bug: #1697129
2 files changed