Default to XFS for rhel7/centos7

This is the default filesystem for both distros, and using ext4
creates some weird mismatches in things like fstab.  I've also hit
some ext4 resize bugs that this will eliminate, at the risk of
exposing new and different xfs bugs.

I didn't want to commit this directly so I could get input about
whether this is a change we want to make at this point in the
release process.  I tested that this fixes my compute image
kernel panics, but I haven't done a full xfs-based deployment.