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
(cherry picked from commit b78eea6146145793a7c61705a1602cf5e9ac3d3a)
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable