Mock out all RPC calls with a fixture

Mock out the rpc proxy calls used by various agents to
prevent unit tests from blocking for 10+ seconds while waiting
for a timeout. This happened with the OVS agent unit tests
recently in Change-ID Idd770a85a9eabff112d9613e75d8bb524020234a.

This change results in a reduction from 330.8 seconds to 2.7 seconds
for the neutron.tests.unit.openvswitch.test_ovs_neutron_agent
test module.

Closes-Bug: #1372076
Change-Id: I5e6794dc33c64c8fe309d8e72a8af3385c7d4442
Merge "Mock out all RPC calls with a fixture"