Remove and recreate interface if already exists

If a nova-compute machine restarts when openvswitch comes up it logs the
following warning messages for all tap interfaces that do not exist:

bridge|WARN|could not open network device tap2cf7dbad-9d (No such device)

Once the compute-node starts it recreates the interfaces and re-adds them
to the ovs-bridge. Unfortunately, ovs does not reinitialize the interfaces
as they are already in ovsdb and does not assign them a ofport number.

This situation corrects itself though the next time a port is added to the
ovs-bridge which is why no one has probably noticed this issue till now.

In order to correct this we should first remove interface that exist and
then readd them.

Change-Id: I1dc01604c5857941d9f35eb1e0c4d5103a56d57b
Closes-bug: #1270973
Related-bug: #1268762
Merge "Remove and recreate interface if already exists"