Tighten up parsing of neutron tap device for ironic

The current parsing of the interfaces from the neutron network namespace
fails if there are other things that match 'tap' (ie, gretaps created
for other purposes)  This tightens up the parsing to only match devices
starting 'tap' instead of anything containing 'tap'

Change-Id: I9a31ec8ad253da0b3c5bd7f5eb105c49850f3060
Merge "Tighten up parsing of neutron tap device for ironic"