Make config_drive_format configurable.

Originally reported in https://github.com/rcbops-cookbooks/nova/pull/377 by
@cbuben, this makes config_drive_format configurable and defaults to 
iso9660.

References https://github.com/rcbops/chef-cookbooks/issues/801
References https://github.com/rcbops/chef-cookbooks/issues/802
Closes https://github.com/rcbops/chef-cookbooks/issues/800