feat(postgres-backup-s3): update AWS CLI to v2; allow support for mul…

[opichon]

…tiple databases; allow support for defining sensitive env vars in files (eg Docker secrets)

AWS CLI updated to v2; python removed; glibc added Added support for multiple databases (note POSTGRES_DATABASE env var is deprecated and replaced by POSTGRES_DATABASES) -- this means this version of the docker image is not backward-compatible with previosu releases. Added support for _FILE env vars for sensitive values. This allows using the image with Docker secrets.

For example,

service:
  postgres-backup
    environment:
      POSTGRES_PASSWORD_FILE=/run/secrets/postgres-password
      ...
    secrets:
      - postgres-password

[iloveitaly]

@opichon could you rebase this on master so I could take a look?