Add dedicated S3 environment variables for weacast loaders

[claustres]

Now all global S3 env vars are forwarded to weacast loaders so that they will automatically archive data when they are defined. This can be a problem if these env vars are defined in the environment for another purpose than weacast data archiving. We should at least create a dedicated S3_BUCKET env var for loaders like we do for Kano (S3 access/secret key is then shared) or create a dedicated env var for each one.