Fix binding of Cfg enties

The default metadata currently prevent the successful binding of Cfg entries:

  * The first patch fixes the traceback of lxml (the traceback is not visible without patching the source, only the message of the exception is shown in the log).
  * The second patch fixes the parsing of the default owner and default group for the files. I use the numeric id 0 to specify the default group, because it is called different on the different host. This would be parsed as False with the default parser. The patch is not optimal because the username and groupname parser resolves the userid and groupid on the host and use the numeric id on the client. Maybe we therefore need a raw parser that simply gets the value as string.
This will require the default user to exist on the Bcfg2 server, which may not necessarily be the case. It will also always translate the username to a UID, which may not be consistent between the server and all clients. This option must allow arbitrary strings.