Clean out namespaces even if we don't delete namespaces

Even when we don't enable namespace deletion, we still want to run the
code that cleans out the namespaces so that the devices get unplugged,
etc.  Otherwise, routers deleted while the agent is down will continue
to operate as if they were never deleted.

The trade-off to consider here is that if there are many stale
namespaces this will slow down the restart of the L3 agent.  The best
option is to get namespace deletion working correctly.  However, where
that has not been worked out yet, this patch provides the cleaning
service for deleted routers.

Change-Id: Ic7b4608a23c4d9530f521d5faff3f8526200b92e
Closes-Bug: #1301042
Related-Bug: #1052535
Merge "Clean out namespaces even if we don't delete namespaces"