stack/slapos: version down zc.buildout 2.7.1+slapos016 slapos.libnetworkcache 0.22 on python3

This partially reverts ee733f9d because
of issues with python3

stack/slapos.cfg

@@ -256,6 +256,11 @@ distro = 1.7.0
 Werkzeug = 1.0.1
 distro = 1.6.0
 
+# temporary revert the version up because of issues with python3
+[versions:python3]
+zc.buildout = 2.7.1+slapos016
+slapos.libnetworkcache = 0.22
+
 
 [networkcache]
 download-cache-url = http://shacache.nxdcdn.com