Added attribute bind_ip to enable configuration of listening ip address

Hi,

just discovered that MongoDB comes listening to all audiences and with no authentication configured when using your cookbook. Had a couple of options ranging from mentioning this in README.md to make use of this cookbook only in trusted environments to integration of MongoDB's basic auth mechanisms.
Decided to go the way of small efforts and integrated the bind_ip directive. Default is: node[:ipaddress] which doesn't change much, but can be easily overwritten within roles e.g. to localhost.

Didn't really get what the mongodb_instance definition at the beginning of mongodb/definitions/mongodb.rb was ment for, so I've decided to add :bind_ip => "0.0.0.0" to not break anything.

Tested on Debian Squeeze with a couple of chef runs, works fine for me.


Regards,
Mike