Add configuration for different backend identity types

Until Keystone has a means of querying the backend identity system (LDAP, keystone's built-in user/project system, etc) which is being used, Horizon should add a setting to configure the identity backend so that we can control behavior such as whether or not users/projects can be updated, users can change their own passwords, etc.

This may require more than simply setting the "type" of backed (e.g. LDAP) but instead having a dict-config-style set of capabilities for the backend (e.g. {"can_add_users": True, ...}).