Ability to start MongoDB using --replSet

[jdoklovic]

I needed the ability to start mongo with a replSet. This PR adds the basic ability to do so.

[mindojo-victor]

+1

  File "/home/jenkins/workspace/***", line 161, in ***
    self.save(write_concern={'w': 'majority'})  # w: "majority" implies j: true
  File "/home/jenkins/workspace/***", line 155, in save
    return super(Document, self).save(*args, **kwargs)
  File "/home/jenkins/workspace/***/venv/local/lib/python2.7/site-packages/mongoengine/document.py", line 294, in save
    raise OperationError(message % unicode(err))
OperationError: Could not save document (norepl)
ERROR

[mindojo-victor]

Although I upgraded to 3.0.9 from 2.4.9 and it worked.