Add option mount_spec to "block-device mount"

Hi, Minio recommends to mount drives using Labels instead of UUID, as such I would like to request an option to override curtin’s default behavior where if no “spec” is specified it defaults to UUID, I do see spec option is already being used for special devices maybe this can be
implemented for normal devices as well maas/src/maasserver/preseed_storage.py at 24ddf01a9bb4f486fb765745d79c77e65eea05cf · canonical/maas · GitHub.

Some discussion about that for more context/info Custom storage mounting

Regards,
Jeff