When building the images we should not be including any values in the
resolv.conf. This change adds the element to clear it for the
overcloud-full, hardened and realtime compute images.
Depends-On: If8c44137d120059b356675a074e3af23ea63a6f8
Change-Id: Ib2531e0a696915ab980aea5a65a425789938786c
Closes-Bug: #1749756