Handle preexiting LVG

[Tcharl]

Hi Libvirt and hpc pros!

The way VG is implemented in this role is kind of limitating: the sacrifice of a physical harddisk is mandatory if we want to use this role with VMs backed by lvm2.
What about letting the user is they want to configure their VG another way, (for example using the mr smith manage lvm role where @markgoddard reviewed my PR two days ago) or use it the simple way.

For example, disabling creation when the 'pvs' attribute is not set, this role can consider that the VG is already setup by the operator