Add cuttlefish_generator:minimal_map/2.

[seancribbs]

This function generates an Erlang configuration based on the passed schema that only includes effects from the passed configuration. That is, defaults in the schema will not be injected and translations for unspecified settings will not be run.

This allows the caller to coerce, validate and translate incoming settings for updates to partial configurations, perhaps as part of a running node.

[andrewjstone]

:+1: de69c8a Code looks good, tests pass, works in my code as well :)

[seancribbs]

@borshop merge