Define default POLICY_DIRS value

Having the default value {} for POLICY_DIRS is useful
for horizon plugin maintainers.
Horizon plugins can add a setting like:

  POLICY_DIRS['network'] = 'neutron_policy.d'

and put their policy files in the policy.d directory.

However, the default value is not set, they need to check
POLICY_DIRS exists to avoid overriding existing POLICY_DIRS.
This is a bit painful.

Change-Id: Ic79ebce34b619efc55675d4da6d04d9955684436

openstack_dashboard/settings.py

@@ -263,6 +263,7 @@ POLICY_FILES = {
     'image': 'glance_policy.json',
     'network': 'neutron_policy.json',
 }
+POLICY_DIRS = {}
 
 SECRET_KEY = None
 LOCAL_PATH = None