Currently, you cannot deploy custom policy files in horizon for disabled
openstack projects that don't have a horizon plugin.
This patch allows customizing disabled openstack projects policy files
used by horizon. For services that do not have horizon plugins, it
creates variables of the form `enable_*_horizon_policy_file` and sets
these to the relevant `enable_*` variables by default. This ensures
backwards compatibility.
This patch is useful when you are migrating to kolla-ansible from
another deployment method one openstack project at a time and you have
custom policy files deployed by the old method.
Partially-Implements: blueprint docs-migrating-to-kolla
Closes-Bug: #1809314
Change-Id: Ifc33ef65759fa4831c24d177b272af5b45f80931