Bugfix: Bring back the USERS_LDAP_USER_SCHEMA_ID variable

We reintroduced the USERS_LDAP_USER_SCHEMA_ID variable which was accidently removed from the users service
with the 4.0.0 release.

https://github.com/owncloud/ocis/issues/7312
https://github.com/owncloud/ocis-charts/issues/397
