In meta data make ACI attributes lower-case, sorted. Add possible attributes.

The metadata contains a list of possible attributes that an ACI for that
object might need. Add a new variable to hold possible objectclasses for
optional elements (like posixGroup for groups).

To make the list easier to handle sort it and make it all lower-case.

Fix a couple of missed camel-case attributes in the default ACI list.

ticket 641

ipalib/plugins/group.py

@@ -81,6 +81,7 @@ class group(LDAPObject):
     object_name_plural = 'groups'
     object_class = ['ipausergroup']
     object_class_config = 'ipagroupobjectclasses'
+    possible_objectclasses = ['posixGroup', 'mepManagedEntry']
     search_attributes_config = 'ipagroupsearchfields'
     default_attributes = [
         'cn', 'description', 'gidnumber', 'member', 'memberof',