Fixing add_instance method

Adding add_instance back to the model, seems I forgot to port it
from the collection earlier.

Fixes fog#308

lib/fog/compute/google/models/instance_group.rb

@@ -33,6 +33,10 @@ def destroy(_async = true)
           service.delete_instance_group(name, zone_name)
         end
 
+        def add_instance instance_id
+          add_instances [instance_id]
+        end
+
         def add_instances(instances)
           requires :identity, :zone