Fix single policy rule handling

With commit 43e9df85ab the handling
of single policy rules was broken and always returned True for a
single rule.

One of the visible results is that the Admin Dashboard showed up
incorrectly for users that lacked permission to see it. Additionally,
panel in the Admin Dashboard were also visible.

This patch fixes single rule handling, and the visible effects.

Closes-Bug: #1653792
Change-Id: I0c8a0d7b230b6c6b7ee048af84646ca95daee340

horizon/base.py

@@ -153,7 +153,7 @@ class HorizonComponent(object):
             for rule in self.policy_rules:
                 rule_param = rule
                 if not any(isinstance(r, (list, tuple)) for r in rule):
-                    rule_param = list(rule)
+                    rule_param = (rule,)
                 if policy_check(rule_param, request):
                     return True
             return False