add mongodb json validators to DAS source code

* validators for dasmaps, inputvals, keylearning
* schema.py is external library. for now I include it here.
  - it could be also useful for nicely validating script input parameters
use json.dumps instead print(dict) to output verification token

* otherwise json.loads would fail (mongoimport was OK)
update dasmaps validator: inputvals, verif_token

* add a record defining how input values can be fetched from given service
* add global verification_token
add validation of dasmaps & metadata upon importing

* validate the dasmaps & metadata before running the actual import
  - this is run by default in das_database_update which runs the update right now
  - it shall also be run before staging the changes to be updated by cron watching the stagedir (without any changes)
minor cleanup & refactoring

* validation of keylearning/inputvals as separate function
* minor code cleanup, pylint fixes
Merge pull request #4136 from dmwm/das-maps-autoupdate

Das maps&metadata validation