[helm] document how to add database

[jeanpommier]

Document how to add a DB config at deployment time, on helm chart.

Should be fairly consistent with https://github.com/apache/superset/blob/master/helm/superset/values.yaml#L111, but I noticed that exported DB config is slightly different, e.g.:

database_name: gis
sqlalchemy_uri: postgresql+psycopg2://superset:XXXXXXXXXX@postgres_host:5432/mydb
cache_timeout: null
expose_in_sqllab: true
allow_run_async: false
allow_ctas: false
allow_cvas: false
allow_dml: false
allow_csv_upload: false
extra:
  allows_virtual_table_explore: true
  metadata_params: {}
  engine_params: {}
  metadata_cache_timeout: {}
  schemas_allowed_for_csv_upload: []
impersonate_user: false
uuid: cab2045e-f58a-4453-bb80-fb97b28cd9ef
version: 1.0.0

[jeanpommier]

I've noticed this issue, which is quite serious. Not advised for now to import databases this way

According to Superset's doc it should be possible to rely on env vars to provide the passwords, which would make it secure again. But my first experiments are not conclusive. My first impression is that the behaviour of SQLALCHEMY_CUSTOM_PASSWORD_STORE is broken (the password definition is replaced by XXXXXX, so we loose the reference to an env var).
It might be related to the issue mentioned above.
I believe the error to be around https://github.com/georchestra/superset-core/blob/prefix_and_header/superset/models/core.py#L383