Mock agent RPC for FWaaS tests to delete DB objs

This changes the firewall service unit tests to
mock the RPC calls from the firewall service to
the agent. This allows the tests to fake the
agent response RPC that removes the firewall
from the DB so the all of the other objects that
the firewall depends on can be deleted.

Closes-Bug: #1288441
Change-Id: I32462ab5557b9c52328bf6a23a12efc2d799644c
Ensure to count firewalls in target tenant

Previously admin tenant cannot create a firewall if other tenant
already created a firewall. We need to count firewalls only in
a target tenant.

Change-Id: I3e6d151d00d4a487bdd858e94929fab8960511a2
Closes-Bug: #1258438
Merge "Mock agent RPC for FWaaS tests to delete DB objs"
Merge "Ensure to count firewalls in target tenant"