Run fixfiles restore in chroot instead of firstboot.

Boot time was 30 seconds shorter in an all-in-one devstack
environment.

Thanks to Ghe Rivero for a pointer to the solution
https://bugzilla.redhat.com/show_bug.cgi?id=208275

Change-Id: I90d0c96d5659326ba67d6119b96d9a4113adf7fe
Merge "Run fixfiles restore in chroot instead of firstboot."