Prompted by this discussion: https://lists.freedesktop.org/archives/systemd-devel/2020-June/044669.html
Strictly speaking this is a compat breakage, but given the tool was added only in the last release, let's try to sail under the radar, and fix this early before anyone notices it wasn't supported always.
Add a option to provide a UUID for the partition that will get created and document that.
Closes #14716.