Using usual way to specify prebuild egg name.

The name should not be set directly.
It is much safer to have the prebuild egg name acquired from the proper
section.
 i.e. (${section-name:egg-name})

component/slapos/buildout.cfg

@@ -39,7 +39,7 @@ python = python2.7
 recipe = z3c.recipe.scripts
 python = python2.7
 eggs =
-  M2Crypto
+  ${M2Crypto:egg}
   slapos.libnetworkcache
   zc.buildout
   ${lxml-python:egg}