software/buildout-testing: make python3 the default

software.cfg becomes python 3
software-py3.cfg no longer exists
software-py2.cfg is introduced

Test suites will have to be updated accordingly

software/buildout-testing/software-py3.cfg → software/buildout-testing/software-py2.cfg

@@ -3,4 +3,4 @@ extends =
   software.cfg
 
 [python]
-part = python3
+part = python2.7

software/buildout-testing/software.cfg

@@ -6,9 +6,6 @@ parts =
   slapos-cookbook
   template
 
-[python]
-part = python2.7
-
 [slapos.buildout-repository]
 recipe = slapos.recipe.build:gitclone
 repository = https://lab.nexedi.com/nexedi/slapos.buildout.git