toolbox: should be blocking (f39)

I marked toolbox as failable when we were getting everthing working on
it (and so we would still have composes). Now that it's working, lets
mark it blocking again.

Signed-off-by: Kevin Fenzi <kevin@scrye.com>

fedora.conf

@@ -256,7 +256,7 @@ image_build = {
                     'repo': 'Everything',
                     'install_tree_from': 'Everything',
                     'subvariant': 'Container_Toolbox',
-                    'failable': ['*'],
+                    'failable': [''],
                     },
             'factory-parameters': {
                 'dockerversion': "1.10.1",