Avoid RunTimeError (dictionary changed size during iteration) with keys()

Fixes #24573

salt/cloud/__init__.py

@@ -228,7 +228,7 @@ class CloudClient(object):
                          if a.get('provider', '')]
             if providers:
                 _providers = opts.get('providers', {})
-                for provider in six.iterkeys(_providers):
+                for provider in _providers.keys():
                     if provider not in providers:
                         _providers.pop(provider)
         return opts