The purpose of this change is to provide a better and more structured
way to define `gitlab_schemas` and `database_connections`.

This helps to define applied behavior for connections,
and what schemas the connection holds, and how those
are configured (load balancing, etc.).

- Each DB and schema is named, and contains description.
- Remove code duplication in `gitlab_schema` EE specs
- Make `Database.db_config_names` to require passing `gitlab_schema`
  to ensure that we return configs only having this schema
- Ensure that database connection order is the same
  as in `config/database.yml`

Changelog: added