introduce read-only account and anonymous_read flag for domain
relocate ldap::defines to ldap::server
simplify relationsship declrations with chaining operators
syntax changes and comment rephrasings
refactoring:

  * only support openldap
  * rename module to openldap
  * use the concat module for file fragments
  * fix acl define and rename to access
  * add syncrepl define
  * add overlay define