Change the rootfs label in F18 and F19

The images produced by the diskimage-builder have their filesystem with
a label of "cloudimg-rootfs", The regex changing fstab on fedora didn't
match the F19 case.

This regex matches both
F18 :
LABEL=_/   /         ext4    defaults        1 1
F19 :
UUID=f2da0fc8-2e12-4a6e-8e95-a31a474aa1a8 / ext4 defaults,noatime 0 0

Change-Id: Iafcf5e069199f1df76874309b2d5064f343fb5e5