Workaround issue in CentOS cloud images with resolv.conf

The CentOS cloud images from 7.2 (1511) onwards have a bogus name server
entry in /etc/resolv.conf, 10.0.2.3. Cloud-init only appends name server
entries to this file, and will not remove this bogus entry. Typically this
leads to a delay of around 30 seconds when connecting via SSH, due to a
timeout in NSS. The workaround employed here is to remove this bogus entry
from the image using virt-customize, if it exists. See
https://bugs.centos.org/view.php?id=14369.

Fixes: #112