NC_ env variable overrides were not appearing in
the output of `occ config:list system` nor `occ
config:system:get xxx`. This was creating nearly
impossible to diagnose configuration/ behavior
disprepancies.

- Refactored readData() so that we aren't saving
  the entire environment in the envCache anymore
  (only those prefixed "NC_") and so that we save
  NC_ provided config values under their real
  key.
- Refactored getValue() to accommodate readData()
  refactor
- Fixed getKeys() to properly return
  envCache keys too

Environment provided config variables now appear
in `occ config:list system` as expected.

Environment provided config variables now appear
when queried via `occ config:system:get KEY`

envCache is now free of non-NC stuff.

Signed-off-by: Josh Richards <[email protected]>

Signed-off-by: Josh Richards <[email protected]>

Signed-off-by: Josh Richards <[email protected]>

Signed-off-by: Robin Appelman <[email protected]>