Support fetching specific user

It is quite like `list_users` so I don't think it needs to be tested but let me know if you want to have tests for this as well.

Without this, you have to fetch all users then filter the one that you need.
You should inherit from `[:openstack, :common, :v2, :public, :base]` so that you can use `base_url` instead of re-implementing its logic inside `def url...end` below. See https://github.com/aviator/aviator/blob/master/lib/aviator/openstack/identity/requests/v2/public/list_tenants.rb for an example.