Added attribute bind_ip.

This enables configuration of mongodb's listening ip address. Default
is: node[:ipaddress] which doesn't change much and can be easily
overwritten within roles e.g. to localhost.
Making 'bind_ip' parameter optional.

Setting 'bind_ip' default value to nil, so we don't change MongoDB
defaults. Adding 'bind_ip' to metadata.
Typo error s/bin_ip/bind_ip/
Merge pull request #39 from rafaduran/master

Adding 'bind_ip' as optional parameter