stack/slapos: Use https pypi index

because http is no longer allowed

https://mail.python.org/pipermail/distutils-sig/2017-October/031712.html

stack/slapos.cfg

@@ -57,6 +57,9 @@ allow-hosts +=
   www.dabeaz.com
   www.owlfish.com
 
+# Use an https index
+index = https://pypi.python.org/simple/
+
 # XXX: Workaround of SlapOS limitation
 # Unzippig of eggs is required, as SlapOS do not yet provide nicely working
 # development / fast switching environment for whole software