Change UUID of root volume on first boot

[andsens]

When debugging an instance, mounting the volume on another instance is the best way to go. This is a problem when the instance used for debugging and the instance to debug use the same AMI.
The system will complain that the UUID of the root volume and the one of the buggy instance is the same. This is an unnecessary problem people have to fix by googling how to override that check.

By changing the UUID on first boot, this problem can be avoided.

[andsens]

This can not be done on XFS since xfs_admin requires the volume to be unmounted.
We can do it on ext* though.