Cloned from launchpad blueprint https://blueprints.launchpad.net/keystone/+spec/email-attribute.
Email is a common attribute of users, and we already explicitly support this
in keystoneclient, as well as show it in the entity examples in the Identity
API specification. We also support mapping to it in our LDAP backend driver.
However, we do not list it as supported attribute in the user resource
Specification URL (additional information):