Add OVS status and fix OVS crash

OVS crash/restart is unpredictable, so neutron-ovs-agent should be
robust enough under that situation. But currently ovs-agent doesn't
figure out this error status(only check ovs restart/normal status)
and still continue to apply subsequent operations(set br/add patch
port/...) till causing exceptions/crash. Add flag to fully represent
ovs status. Base on that, we can add proper fail-over code in method
rpc_loop, to treat ovs dead/restart gracefully to prevent agent
crashes while it is running.
Closes-bug: #1296202

Change-Id: Id058b7ddef2ed337627dc692d0418786ad14f4b4
Merge "Add OVS status and fix OVS crash"