Fix /etc/openstack-dashboard/ permissions for access to *policy.json

Fix /etc/openstack-dashboard/ permissions for access to *policy.json

The Kolla Dockerfile sets the permissions for /etc/openstack-dashboard/
to horizon:horizon. We need this to be readable by the apache user
as the horizon user is not the user in which httpd runs with. We may
want to consider fixing this in the upstream Dockerfile instead, e.g.
checking if we're using centos/rhel and changing the permissions that
way. I'm not sure why it's set to horizon:horizon upstream, and I'm keen
not to break any existing functionality that relies on the horizon based
permissions.

Closes-Bug: #1723125
Change-Id: If5feebae38f7fdfffa60bfaedc4521f676006484
(cherry picked from commit fd657aa4e68de7ad239a88525b5ae343acd3bf80)