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 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). From there, there 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 objects or tenancies.