Refactor LDAP variable names

Some names were not very descriptive:
- Why were user and group relative DNs shoved into a single array?
- Why use "cn" for username lookup? It is not unique by definition.
- "ldap_admin_cn" was named a little unfortunately, as it's a group.
- The same goes for "ldap_user_cn", which was named so to be conform
  with "ldap_admin_cn".

I understand refactoring is sometimes frowned upon, so I saved this
commit for last.