Fix the migration adding a UC to agents table

The migration script mistakenly assumes that all core
plugins use agents extension, which is not true (e.g.
plumgrid and bigswitch don't).

Apply this migration script only for plugins that are
stated in the original migration script adding agents
table (511471cc46b_agent_ext_model_supp.py).

Related-Bug: #1254246

Change-Id: I7915ef8d183782eb5d46ac47f45014aa9e9640fb
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 "Fix the migration adding a UC to agents table"