openvswitch: delete ports intelligently

openvswitch: delete ports intelligently

So far, when creating veth devices attached to openvswitch bridges we used to
fork() off a thread on container startup. This thread was kept around until the
container shut down. I have no good explanation why we did it that why but it's
certainly not necessary. Instead, let's fork() off the thread on container
shutdown to delete the veth.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>