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.
Export FS_TYPE in instack-virt-setup

I don't think it will be respected otherwise.
Merge branch 'master' of https://github.com/cybertron/instack-undercloud into cybertron-master

Conflicts:
	scripts/instack-build-images
Merge branch 'cybertron-master'