Support default acl_dir for acl_files.

Just about all of the files are of the form $acl_dir/$project_name. That's
a whole load of repitition, when we can provide a sensible overridable default.

Change-Id: Ib17e21851978972205bc3342347a72cf9b078c6c
Reviewed-on: https://review.openstack.org/23369
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins