net_tests: add reusing dhclient owned iface

https://bugzilla.redhat.com/1100264 showed us that we are lacking
testing of interface reusing, specifically about reusing dhclient
owned devices. This patch covers this hole in our test coverage.

Change-Id: I5ceb7dcc853d2d2683c36327143175795de56854
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28049
Reviewed-by: Dan Kenigsberg <danken@redhat.com>