[ops-guide] Add keystone v3 / domains documentation

[Gauvain Pocentek]

Identity v3 brings several interesting features to ops, that probably have their place in the ops-guide.

A non-exhaustive list:

- use groups to ease user/project association
- multiple domains to separate real users (LDAP/AD) from system users (service tenant)
- give admin responsibilities to some users in domains
- federation

[Frank Kloeker (f-kloeker)]

created: https://storyboard.openstack.org/#!/story/2005742