If your mongodb driver supports connection with username/password, then use OPENSHIFT_MONGODB_DB_URL instead of OPENSHIFT_MONGODB_DB_HOST
OPENSHIFT_MONGODB_DB_URL gives you this format:
mongodb://admin:password@127.4.99.1:27017/
and OPENSHIFT_MONGODB_DB_HOST gives you this format:
ip addres, ex: 127.4.99.1
So you can just use OPENSHIFT_MONGODB_DB_URL to connect and authenticate at the same time
with mongoskin, you can just do this:
var db = require('mongoskin').db(process.env.OPENSHIFT_MONGODB_DB_URL + 'dbname'+ '?auto_reconnect=true',
{safe: true, strict: false}
);