Use object creation templates for creation of users, tenants, etc.

Rather than hardcoding the objectclasses and attributes that will be used for user, tenant, role, etc. creation, we should use templates, where keystone administrators could specify how they'd like the objects created. For instance, here's a YAML example for a user:

user_template:
  objectclasses:
    - inetorgperson
    - posixuser
    - top
  attributes:
    - uid: {{ id }}
    - cn: {{ name }}
    - sn: {{ id }}
    - uidNumber: {{ extension:posix:uidnumber }}
    - homeDirectory: /home/{{ id }}
    - accountShell: /bin/bash

Template files would be placed in /etc/keystone/templates. Required extended variables would need to be made known to clients in some way.