print usage when calling das_update_database w/o params

related to pull #4128
remove duplicated dasmap management scripts

- remove bin/das_map bin/das_map_admin bin/das_yml2json and tools/das_mapping_db.py
  * the functionality is duplicated by scripts used to export dasmaps as mongodb json dumps and import them
  * updated the documentation accordingly

fix issue #4129
renamed das_drop_maps -> bin/das_maps_yml2json

* bin/das_drop_maps -> bin/das_maps_yml2json
* tools/das_drop_maps.py -> tools/das_maps_yml2json.py

fixes issue #4129
added description to bin/das_maps_yml2json
make sure the supplied dasmap dir exists
Merge pull request #4130 from dmwm/db_import_scripts_1

Db import scripts: remove duplicated functionality / refactor