stack/slapos.cfg: pin slapos.toolbox to 0.128 on python2

slapos.toolbox no longer supports python2, this pins to the last version
where the test passed on python2.

In nexedi/slapos.toolbox!116 (comment 181641)
we agreed that if we need to fix a python2 bug or backport something
critical, we can make a version 0.128.1 (based on 0.128 with the fix
for python2)

stack/slapos.cfg

@@ -360,6 +360,7 @@ pyparsing = 2.2.0
 pyrsistent = 0.16.1
 requests = 2.27.1
 selectors34 = 1.2
+slapos.toolbox = 0.128
 smmap = 0.9.0
 smmap2 = 2.0.5
 traitlets = 4.3.3