software/slapos-sr-testing: Use zc.buildout=2.7.1+slapos020

zc.buildout=2.7.1+slapos020 is SlapOS-patched buildout2 with support
for bootstrapping buildout3, required to be able to install buildout3.

Downgrade related eggs to pre-buildout3 versions as well.

software/slapos-sr-testing/software.cfg

@@ -503,3 +503,11 @@ xmltodict = 0.13.0
 
 # Patched eggs
 PyPDF2 = 1.26.0+SlapOSPatched001
+
+# Buildout bootstrap versions
+zc.buildout = 2.7.1+slapos020
+zc.recipe.egg = 2.0.3+slapos003
+slapos.rebootstrap = 4.5
+slapos.recipe.build = 0.56
+slapos.recipe.cmmi = 0.19
+setuptools = 44.1.1