Add flows to tunnel bridge with proper cookie.

Add flows to tunnel bridge with proper cookie.

Without that fix flows applied to br-tun through
DeferredOVSBridge are created without cookie.
That results in l2pop flows being deleted in the process of
cleanup of stale flows.

Solution is to add cookie to all add/mod-flows of OVSBrigde
class in the method do_action_flows.
Also, agent_uuid_stamp moved to a proper place - into the
base OVSBridge class as storing attributes in Mixing was
just a wrong code design.

Change-Id: Ic09a0dbc04fc5da38d30e1392cf2ea27d576040c
Closes-Bug: #1489372