In '_teardown_server' method driver was searching for all matches of
'veth' prefix and looping them for deletion. But using Xenial host
it finds each such veth name twice and tries to delete the same veth
twice too getting following error:
ovs-vsctl: no row "veth7ac5738" in table Interface
So, fix it by translating 'list' of veths to 'set' that
will have only unique names.
Change-Id: I819a885547f9fc595b15f98e4c94de21e33914ac
Closes-Bug: #1630512
(cherry picked from commit 8401c150f3)
5a77f19328