slaprunner: install slapos.core egg explicitly

Solve the problem that `slapos` script is missing from slaprunner ( described in !253 ) by adding `slapos.core` egg explicitly.

/reviewed-on !254

software/slaprunner/common.cfg

@@ -152,6 +152,7 @@ eggs =
   gunicorn==19.7.1
   futures
   ${slapos-cookbook:eggs}
+  slapos.core # listed explicitly for scripts generation
 
 [extra-eggs]
 recipe = zc.recipe.egg