Initial LDAP group member iteration support

Add interface for group member iteration on internal auth providers and implement support in the LDAP interface.

data/users/__init__.py

@@ -186,6 +186,16 @@ class UserAuthentication(object):
     """ Verifies that the given username and password credentials are valid. """
     return self.state.verify_credentials(username_or_email, password)
 
+  def iterate_group_members(self, group_lookup_args, page_size=None, disable_pagination=False):
+    """ Returns a tuple of an iterator over all the members of the group matching the given lookup
+        args dictionary, or the error that occurred if the initial call failed or is unsupported.
+        The format of the lookup args dictionary is specific to the implementation.
+        Each result in the iterator is a tuple of (UserInformation, error_message), and only
+        one will be not-None.
+    """
+    return self.state.iterate_group_members(group_lookup_args, page_size=page_size,
+                                            disable_pagination=disable_pagination)
+
   def verify_and_link_user(self, username_or_email, password, basic_auth=False):
     """ Verifies that the given username and password credentials are valid and, if so,
         creates or links the database user to the federated identity. """