Remove and recreate interface if already exists

If the dhcp-agent 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 dhcp-agent 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.

Closes-bug: #1268762

Change-Id: I4bb0019135ab7fa7cdfa6d5db3bff6eafe22fc85
Merge branch 'master' of git://git.openstack.org/openstack/neutron into odl_ml2
Fix unneeded whitespace issues.

Change-Id: I41e7e4b355209c638d769fcefad89646a2509170