Separate python from host system.

Thanks to using own python with own libraries, there will be library version
conflicts, as lxml is built against own libz.

component/slapos/buildout.cfg

 [buildout]
 extends =
   ../lxml-python/buildout.cfg
+  ../python-2.7/buildout.cfg
 
 parts =
   slapos
@@ -10,8 +11,12 @@ find-links =
 
 versions = versions
 
+[lxml-python]
+python = python2.7
+
 [slapos]
 recipe = z3c.recipe.scripts
+python = python2.7
 eggs =
   ${lxml-python:egg}
   slapos.core