Add catch-call try/catch within rpc_loop in ovs plugin agent

Add catch-call try/catch within rpc_loop in ovs plugin agent

related to bug 1050512

when running in db-mode, the ovs plugin agent will catch any unexpected
exceptions generated during processing.  However, in rpc-mode, this
does not happen, meaning a small error, even a transient one, causes the
agent to exit completely.  Thic change adds a try-catch block to the
rcp_loop(), causing the agent to log any unexpected exception, wait for
the polling period, then retry the loop after resetting all state.

Change-Id: I76eae1800831e59c5078c4be8fa5ca22298bfb0a