Fix AttributeError during startup of ovs agent in DVR mode

Fix AttributeError during startup of ovs agent in DVR mode

Make sure the agent starts processing incoming requests only after the entire
initialization is complete. This is done by making explicit when the rpc loop
is supposed to start, i.e. right at the end of the init process.

This fix was necessary because the agent starts processing rpc messages even
though it has not completed the entire initialization of bridges and data
structures; this is usually okay, but in case of DVR, this leads to a
situation where during the the first run, the agent asks the server to be
assigned a MAC address; this in turn leads the server to fanout the generated
MAC to the running agents, the requesting one included; because of the
incomplete setup, the above mentioned error occurs. During subsequent
restarts, the problem no longer appears.

Closes-bug: #1395196

Change-Id: I792697b94fef39971693cf8aff715c270601cecb