zero out resolv.conf during install
Anaconda is writing an /etc/resolv.conf from the install environment.
The system should start out with an empty file, otherwise cloud-init
will try to use this information and may error:
https://bugs.launchpad.net/cloud-init/+bug/1670052
(cherry picked from commit fc0a635bc48a0293c43da01207e9095039198911)