Does not support multiple mongodb units

[Marius B. Kotsbak]

If multiple mongodb units are deployed, the node app should probably try to connect with more than one of them, in case that one dies, like this:

"MongoClient.connect("mongodb://localhost:30000,localhost:30001/integration_test_?w=0&readPreference=secondary", function(err, db) {"

Currently, the charm just uses the last associated unit.