Fixed errors/warnings related with "mongodb:start" command

1. I'm getting the following warning after upgrading to Docker 0.9.0:
"Warning: '-name' is deprecated, it will be removed soon. See usage."
I've updated the "run" command option to fix this warning.
2. Running "mongodb:start" after "mongodb:stop" gives the following error:
"Error: Conflict, The name dokku-mongodb is already assigned to CONTAINERID. You have to delete (or rename) that container to be able to assign dokku-mongodb to a container again."
I've changed code to create new MongoDB container (using "run") only if it does not exist.