consistent route for metadata creation across ETL rules

[thomas-fred]

In the ETL process there are 3 rules which create metadata entries, each called something like POST_metadata_to_backend. 1 for ISIMIP, 1 for Aqueduct and 1 for every other dataset.

I had stripped out the leading /api from /api/tiles/sources for the dataset specific routes used to create metadata for the dataset specific routes for a particular configuration of services locally. This was useful if you wanted to direct the request at the backend service directly (say if there is no frontend). It is not ideal when creating metadata entries on a remote, where you are more likely to need to pass through a reverse proxy (which needs the /api for routing to the appropriate service).

Change to have /api/tiles/sources consistently as the route, and change documentation and env variable names to make clear you need a proxy of some kind. This allows ETL to interact with local and remote instances with only configuration changes.