Set fake rpc backend impl for TestLinuxBridgeAgent

Fixes bug 1153845

Change-Id: I692bfc16dbd52dacd2a9418f0bd63c3143a25025
Registers root_helper option for test_iptables_firewall

Fixes bug #1146478

Change-Id: I908a142d9463719e7b3c94aa1d699d329c9259de
Merge "Registers root_helper option for test_iptables_firewall"
Merge "Set fake rpc backend impl for TestLinuxBridgeAgent"