incorrect os-credentials handling

[Dmitrii Shcherbakov]

The charm currently incorrectly handles os-credentials passed to it.

symbolic project_domain_name and project_name are optional with default values hard-coded, however, project_id and project_domain_id are mandatory because they are retrieved unconditionally.

This prevents one from using just symbolic names and is also potentially harmful for the openstack client because it only needs either symbolic names or IDs (a mix is possible but probably not justified).

https://git.launchpad.net/prometheus-openstack-exporter-charm/tree/reactive/openstack_exporter.py?id=eda95420b14d30ceca7df3b67e2b277442005120#n126

    if config_creds_yaml:
        config_creds = yaml.safe_load(config_creds_yaml)
        creds = {
            'credentials_username': config_creds['username'],
            'credentials_password': config_creds['password'],
            'credentials_project': config_creds.get('tenant_name', 'admin'),
            'region': config_creds['region_name'],
            'auth_url': config_creds['auth_url'],
        }
        identity_api_version = config_creds.get('identity_api_version', 2)
        if identity_api_version == 3:
            creds['credentials_user_domain_name'] = config_creds['user_domain_name']
            creds['credentials_project_name'] = config_creds.get(
                'project_name', 'admin')
            creds['credentials_identity_api_version'] = identity_api_version
            creds['credentials_project_domain_name'] = config_creds.get(
                'project_domain_name', 'admin_domain')
            creds['credentials_project_id'] = config_creds['project_id']
            creds['credentials_project_domain_id'] = config_creds['project_domain_id']

[Andrey Grebennikov (agrebennikov)]

Also Charm description in the store has to be updated with the guidance how to properly use V3 credentials.