vnics: don't presume existence of filterref/filter

With quantum/ovs plugin, there is no necessarily a "filteref" in interface param.
If not present don't lookup the "filter", let it to None and no more use
vnics.fref as resource_id.
resource_id becomes : instance_name + instance_uuid + vnic_name

fix Bug #1180466

Change-Id: I21c15f9ee4bd5303a7f2e287c7fe69aa7d0d5b0a
Merge "vnics: don't presume existence of filterref/filter"