Code review comment for ~smoser/maas:cleanup/render-curtin-config-internally

Actually, what I think i'd do and suggest is to
a.) rename the keys from the names that did exist in that file
  early_commands/driver_* to early_commands/30_driver_*
  debconf_selections/maas_* to debconf_selections/maas_internal

b.) "clean" out the old names if they were present in the loaded config

this would mean that user's modifications to that file that modified *those* things would actually fail to have affect. But generally speaking they should not have done that, or we'd like to have bugs to see what was wrong.

then just warn in log that it is ignoring those things.
that'd keep their file basically working as expected other than some things
in it not actually having any affect.