ldap acl: merge rules inside acl

[wdavidw]

The merge property shall apply to a single acl, exemple:

nikita.acl.add({
  acls: [{
    to: "dn.subtree='ou=users,dc=ryba'",
    by: [
      "dn.children='ou=users,dc=ryba'
    ],
    overwrite: false, Default is to merge
  }]
})