Change the rootfs label in fedora's /etc/fstab.

The images produced by the diskimage-builder have their filesystem with
a label of "cloudimg-rootfs", so we need to change the default /etc/fstab
on fedora to reflect that.

Change-Id: Id1bb00cb81cb200a114f500e26272624be577da0
Merge "Change the rootfs label in fedora's /etc/fstab."