Add *args, **kwargs to ModalBackdropMixin's init method

Before this change ModalBackdropMixin called super(...).__init__()
without arguments. This imposed restrictions on what classes this
mixin could have been mixed into, i.e. only classes, that do not accept
any parameters.
This proved to be a problem for murano-dashboard, since it uses this
mixin (indirectly through ModalFormMixin) and mixes it into a class,
that accepts parameters to it's init method.
This change allows to use ModalBackdropMixin with classes that have
init methods with parameters.

Change-Id: I6155476738021b784ef7e643c968f1d784b15906
Closes-Bug: #1582816
(cherry picked from commit bb1547c8ab)

horizon/forms/views.py

@@ -43,8 +43,8 @@ class ModalBackdropMixin(object):
     """
     modal_backdrop = 'static'
 
-    def __init__(self):
-        super(ModalBackdropMixin, self).__init__()
+    def __init__(self, *args, **kwargs):
+        super(ModalBackdropMixin, self).__init__(*args, **kwargs)
         config = getattr(settings, 'HORIZON_CONFIG', {})
         if 'modal_backdrop' in config:
             self.modal_backdrop = config['modal_backdrop']