Add clock sync error detection on agent registration

Add clock sync error detection on agent registration

For the server to determine if an agent is alive or not,
it depends on the agent's clock being mostly in sync with the server
clock. The neutron-server may reject and return the request if
there's a timestamp difference between the two nodes. Currently
there's no good way to detect this condition from the agent code.

This fix will improve the error handling
logic by writing an appropriate log in the neutron server's log
file for an early detection of the problem.

Change-Id: If884f90c4b1786cfc63d3e2ff2d66f92122258c2
Closes-Bug: #1432582