Allow custom rootfs labels

Per [1], our current root label of cloudimg-root does not work with
XFS because XFS only allows 12 character labels.  This change
addresses that by allowing a custom rootfs label to be specified
in the call to disk-image-create.  There should be no backwards
compatibility concerns as the default label is unchanged.  Any
external elements dealing with the label would need to be updated
to support this new feature, but should continue to work as before
as long as a custom label is not specified.

[1]: https://bugzilla.redhat.com/show_bug.cgi?id=1139584

Change-Id: I596104d1a63b5dc6549e8460a1ae3da00165ef04