Reprogram flows when ovs-vswitchd restarts

When OVS is restarted, by default it will not reprogram flows which were
programmed. For the case of the OVS agent, this means a restart will cause
all traffic to be switched using the NORMAL action. This is undesirable for
a number of reasons, including obvious security reasons.

This change provides a way for the agent to check if a restart of ovs-vswitchd
has happened in the main agent loop. If a restart of ovs-vswitchd is detected,
the agent will run through the setup of the bridges on the host and reprogram
flows for all the ports connected.

DocImpact
This changes adds a new table (table 23) to the integration bridge, with a
single 'drop' flow. This is used to monitor OVS restarts and to reprogram
flows from the agent.

Conflicts:
	neutron/plugins/openvswitch/common/constants.py

Change-Id: If9e07465c43115838de23e12a4e0087c9218cea2
Closes-Bug: #1290486
(cherry picked from commit 8e9f00a19dab98e5cfc7ca32beb9f17ebb5bc1bb)
Merge "Reprogram flows when ovs-vswitchd restarts" into stable/icehouse