Consider clarifying documentation

[myllynen]

Currently the README states:

As of now, the role supports managing file systems and mount entries on

    unpartitioned disks
    lvm (unpartitioned whole-disk physical volumes only)

However, it looks like the role is also able to create new LVs on existing VGs (created earlier perhaps during OS installation).

Additionally, it is not mentioned that disks is optional for storage_pools and examples do not demonstrate such usage.

Perhaps documentation could be clarified a bit in this regard if it is a valid and supported use case for the role to be used in cases without specifying disks.

Thanks.