support identifying existing devices by UUID

[dwlehman]

The current codebase does not make any attempt to identify devices by descriptors other than name. While we cannot specify UUID for new devices, we can support lookup/id of existing devices by a uuid field.

[pcahyna]

If you mean the UUID field in the blkid output, I would say this is a bad idea. This data is derived from the content of the disk, not from the disk itself. And the content of the disk is going to change when the role is applied, making the playbook non-idempotent. It can be a good idea only if you are sure you want to change the attributes of an existing filesystem without reformatting it.

[pcahyna]

Also, why can't we specify it for new devices? mkfs.ext4, mkfs.xfs and mkswap all support it.