Switch openstack drivers to useing show_instance

there isn't a reason to use list_node_full directly if we can just use the show_instance function.

If show_instance also uses list_node_full, then so be it, but we could always make it more efficient for 
some of the other drivers that aren't using libcloud.

This cuts down on the number of connections that the nova driver uses why querying server information because it doesn't have to do a new query for ever each server when doing a server_list_detailed.

@techhat ping