Implement namespace cleanup for new DVR namespaces

DVR introduces a new namespace type called snat-<router_id>.  These
namespaces are not properly cleaned up when found stale after an agent
restart.  This patch fixes that.

Change-Id: I0d8b83e0b7838957742e72aa2f42b15e0ca67f04
Partially-implements: blueprint neutron-ovs-dvr
Merge "Implement namespace cleanup for new DVR namespaces"