Support multiple groups for users

[thomas-vogels]

Description

This PR brings us the option to configure multiple groups for each user.
This allows us to bring in the configuration of users, which we have done before on the command line.

There seem to be a lot of changes here but it's really just pushing the change of "group -> groups" through.

That said, having this PR first would help: Add "groups" and "comment" to user info #591

User-visible changes

The following commands have been updated:

• initialize -- all users and groups from the configuration are created if requested. This now supports users being in multiple groups.
• create_users (or its alias create_user) -- this no longer supports specifying a group or creating a schema for the user. You should specify a user's groups in the configuration file. If you need to create a schema, use update_user.
• update_user -- this no longer supports specifying a group or creating a schema for the user. You should specify a user's groups in the configuration file. You can use this command to add a user's schema. If you have added the user's password to your .pgpass file, then the user's password will be updated.

Note that users are now created without passwords and cannot login until you run:

arthur.py update_user

In order to be backwards compatible with current configuration files, both group with a single identifier and groups with a list oof identifiers is supported to configure a user's groups.

Note that users is a list in the configuration and you cannot spread the definition of users over multiple files. The last list of users in configuration files wins.

Here's an example for user configuration.

{
    "data_warehouse": {
        "users": [
            {
                "name": "default",
                "group": "analyst"
            },
            {
                "name": "tom",
                "groups": ["analyst", "etl_ro"]
            }
        ]
    }
}

Links

Supports: Manage users access and group membership using configuration #581

Testing

You can run all commands using --dry-run to make sure that they would do what you want them to do.