fork child process for processing job.

This patch is to enable forking of child process for processing the
given job. The forking of child process can be enabled or disabled using
the qonos worker config param flag, 'fork_child_process'. By default
fork_child_process is disabled ie., set to 'False'.
_child_pid is initialized to None and only changed to something other than None in the try block. Does this answer your question? If not, hit me up on IRC.