Fix lxml installation for the backup server.

software/backupserver/software.cfg

 [buildout]
 extends =
+  ../../component/lxml-python/buildout.cfg
   ../../component/dash/buildout.cfg
   ../../component/dcron/buildout.cfg
   ../../component/logrotate/buildout.cfg
@@ -15,6 +16,7 @@ extends =
   ../../stack/slapos.cfg
 
 parts =
+  eggs
   rdiff-backup
 #   duplicity
   dcron
@@ -159,6 +161,11 @@ signature-certificate-list =
   m711+GzlW5xe6GyH9SZaGOPAdUbI6JTDwLzEgA==
   -----END CERTIFICATE-----
 
+[eggs]
+recipe = zc.recipe.egg
+eggs =
+  ${lxml-python:egg}
+
 [directory]
 recipe = slapos.cookbook:mkdirectory
 template = ${buildout:directory}/template
@@ -225,6 +232,7 @@ slapos.cookbook = 0.78.3
 slapos.recipe.build = 0.11.6
 slapos.recipe.cmmi = 0.1.1
 slapos.recipe.template = 2.4.2
+zc.recipe.egg = 1.3.2
 
 # Required by:
 # slapos.core==0.35.1