Charms use this relation to obtain keystone credentials without
creating a service catalog entry. Set 'username' only on the relation
and keystone will set defaults and return authentication details.

Possible relation settings:
username: Username to be created.
project: Project (tenant) name to be created. Defaults to services
         project.
requested_roles: Comma delimited list of roles to be created
requested_grants: Comma delimited list of roles to be granted.
                  Defaults to Admin role.
domain: Keystone v3 domain the user will be created in.
        Defaults to the Default domain.

Change-Id: I465d2273560d86752d1bfc7497a9139a9604f814