fix hardcoding pieces in helm chart

[longshuicy]

• different username/password for minio fails, hardcoded username/password in backend?
• allow to configure keycloak Helm deployment setup redirect in keycloak.json file when importing  #191
• setup externalsecrets as a way to not hardcode the secrets in the helm chart.

[lmarini]

See docker-compose for how to initialize realm:

    volumes:
      - ./scripts/keycloak/clowder-realm-prod.json:/opt/keycloak/data/import/realm.json:ro
      - ./scripts/keycloak/clowder-theme/:/opt/keycloak/themes/clowder-theme/:ro
    command:
      - start-dev
      - --http-relative-path /keycloak
      - --import-realm