deploy-test: Pin KVM revision

It is not needed to follow KVM development blindly and be hit by
bugs during the development.

software/erp5testnode/testsuite/deploy-test/software.cfg

 [buildout]
-extends = ../../../kvm/software.cfg
+extends = https://lab.nexedi.com/nexedi/slapos/raw/8deeab28fb7d4fd527c2df05f74fcaf27c8df218/software/kvm/software.cfg
 
 parts =
   eggs