Unification + expansion of the environment variables

[peb-adr]

Please make the following changes to the environment variables:

- remove:
  MEDIA_PRESENTER_HOST
  MEDIA_PRESENTER_PORT
  MEDIA_DATABASE_HOST
  MEDIA_DATABASE_NAME
  MEDIA_DATABASE_PASSWORD_FILE
  MEDIA_DATABASE_PORT
  MEDIA_DATABASE_USER

• add:

  PRESENTER_HOST
  PRESENTER_PORT
  DATABASE_HOST
  DATABASE_NAME
  DATABASE_PASSWORD_FILE
  DATABASE_PORT
  DATABASE_USER

The env vars for the DB connection should be renamed.

Instead of having MEDIA_PRESENTER_HOST, PRESENTER_HOST should be set
for the media service overriding the value from the default env (analogous for
the port).

More details on these changes are in manage-service issue OpenSlides/openslides-manage-service#224.
Also please provide non-service-specific feedback on these related issues in the
manage-service issue.

[jsangmeister]

duplicate of #67 - also, PRESENTER_{HOST,PORT} are already named that way without the prefix.