Allow for physical_volumes and volume_groups that change a system lives

The issue being addressed is the change of a systems after it is provisioned.  Sometimes you have to add disks or modify a server that makes it non standard.  Usually this occurs on systems that have a large amount of data.

By adding the  unless_vg (physical_volume) and createonly (volume_group) options you can have standard deployments and still support the non standard growth while using puppet-lvm.