[FEATURE] overwrite credentials>cred>businessUnits in retrieve/commit/deploy functions

[JoernBerkefeld]

right now one has to manually update .mcdevrc.json whenever there is a new BU.
it would make much more sense if this info came straight from the Environment records involved in retrieve/commit/deploy function executions instead.

we should store the intended bu-folder name on the environment's system properties - otherwise we cannot auto-generate those entries