Enable LDAP as a Keystone backend

This enables (and sets to default, but setting `node['bcpc']['keystone']['backend']` to `'sql'` maintains existing behavior) the LDAP backend for keystone so that all User, Tenant, and Role details are stored there. The recipes use the `node['bcpc']['domain_name']` translated into LDAP-speak as the base DN for the directory (e.g. `bcpc.example.com` becomes `dc=bcpc,dc=example,dc=com`). Below that, the children OU's are `ou=People` for Users, `ou=Tenants` for Tenants, and `ou=Roles` for Roles. This is read/write enabled but could be limited to read-only with flags in `keystone.conf` if there's an external LDAP used for authoritative objects or tenancies.