flag to optionally manage mongos service startup + fix mongos/d shebang for ulimit

there are some occasions where we need to manage the mongos service startup outside of this module (especially when orchestrating environments in EC2, and we need to wait for config servers to start before starting mongos etc). provide a toggle to externally manage it, but configure mongos*.conf using this module still

also, ulimit -u does not work by default with a #!/bin/sh shebang it seems on Ubuntu 12.04
changed to #!/bin/bash as per http://askubuntu.com/questions/97257/ulimit-u-gives-illegal-option-error-when-running-from-a-script-but-not-when-fro and it worked fine.