Make ML2 ensure_dvr_port_binding more robust

There is a remote chance that this operation may
be prone to DB integrity errors, in case the binding
is attempted on the same port twice.

Ideally getter methods should not create, but this
is a common Neutron (anti)-pattern that would be
difficult to eradicate (at least in a single patch);
so for now let's make this code more defensive.

Related-bug: #1269131
Related-bug: #1335226

Change-Id: Ie6c57fd46f0752839814dbac5b14fae2364f973d
Merge "Make ML2 ensure_dvr_port_binding more robust"