Ensure ovsdb-client is stopped when OVS agent dies

If the OVS agent is killed, the interpreter is killed before any cleanup
is done. This patch adds a signal handler for SIGTERM that exits
normally so that the existing cleanup is done and the ovsdb-client
process is terminated.

Change-Id: Ie4f8ff434098100f4699124403d58caa784084cf
Merge "Ensure ovsdb-client is stopped when OVS agent dies"