Exit rpc_loop when SIGTERM is recieved in ovs-agent

Previously ovs-agent exited with exit code 1 after SIGTERM was received.
SIGTERM should shutdown agent gracefully and exit code should be 0.

Closes-Bug: #1334264
Change-Id: I23e81cee5ae9e9fdabbe1377420b0902f47be8a7
(cherry picked from commit 60fc1f64fabd86c38846c76785255c523a98b331)
Merge "Exit rpc_loop when SIGTERM is recieved in ovs-agent" into stable/icehouse