Avoid having None in known_hosts file.

slapos/recipe/dropbear.py

@@ -43,6 +43,7 @@ class KnownHostsFile(dict):
   def _dump(self):
     with open(self._filename, 'w') as keyfile:
       for key, value in self.items():
+        if key is not None and value is not None:
           keyfile.write('%(host)s %(key)s\n' % {'host': key,
                                                 'key': value})