Previously, network namespaces would not work correctly when
restarting or stop/rm/start the neutron-agents container. This
is because network namespaces created within the container are
associated to the container's pid: /pid/$CONTAINER_PID/ns/net.
With this patch, when a container starts or restarts, the l3/dhcp
agent start scripts will remove any existing qrouter/qdhcp network
namespaces before starting the agents. The agents will recreate the
necessary network namespaces since they are stored in the db.
Closes-Bug: 1444219
Change-Id: Ia86729766fe8c2fc145b3a02d519746b149a73bb
17fc2d9b47