List auth domains v3 identity endpoint

This commit adds ``list_auth_domains`` to the v3 identity
client.

Note that the related test doesn't require admin credentials [0]
yet has been created in the admin sub-folder for identity
because ``test_list_auth_projects`` was already present therein.

[0] https://github.com/openstack/keystone/blob/82f60fe22c405829f8e5f6576f25cf3663b10f73/keystone/auth/controllers.py#L410

Change-Id: Ie50c30bfff1900a9056aaedca7ba15b0d9573eab
4 files changed