[identity, common] support domains

So far, the creation of users, roles and projects (tenants) is supported. With the Keystone v3 API, there's the possibility of using "domains": https://answers.launchpad.net/keystone/+question/240600

For this, we need

- a different openstack_command_env, supporting the proper ENV variables,
- python-openstackclient, since it's the only client supporting domains,
- openstack-identity's register provider needs another action, create_domain,
- the other create_user, create_tenant actions need a domain setting.

This blueprint kind of depends on a decision in this blueprint: https://blueprints.launchpad.net/openstack-chef/+spec/cli-client-packages
since, if we decide to go without the python-xxxclient, we might as well do it properly (whatever the discussion yields) here. (Given that, the work items can be created.)