install: fix subclassing of knob groups

Add new @group decorator to declare an installer class as a knob group
instead of subclassing Group, so that subclassing the installer does not
create duplicates of the original group.

https://fedorahosted.org/freeipa/ticket/6392

Reviewed-By: Martin Basti <mbasti@redhat.com>

ipapython/install/core.py

@@ -23,7 +23,7 @@ from . import util, typing
 from .util import from_
 
 __all__ = ['InvalidStateError', 'KnobValueError', 'Property', 'Knob',
-           'Configurable', 'Group', 'Component', 'Composite']
+           'Configurable', 'group', 'Component', 'Composite']
 
 NoneType = type(None)
 
@@ -516,11 +516,14 @@ class Configurable(six.with_metaclass(abc.ABCMeta, object)):
         self.__state = to_state
 
 
-class Group(Configurable):
-    @classmethod
-    def group(cls):
+def group(cls):
+    def group():
         return cls
 
+    cls.group = staticmethod(group)
+
+    return cls
+
 
 class ComponentMeta(util.InnerClassMeta, abc.ABCMeta):
     pass