Fix Openstack Storage service_type

Change the Openstack Storage service (Swift) openstack_service_type default value from "object-store" to ["object-store"].
    
The problem with "object-store" as an openstack_service_type is that if the Swift service is unavailable, the authenticate_v2 method will attempt to call:
    
   service_type.join ", "
    
This will fail for anything that isn't an array.
    
Changing the openstack_service_type to an array solves this problem.  Additionally, it keeps the openstack_service_type consistent with all of the other Openstack services' openstack_service_type default values.

Fixes issue #3112