Fix `mongodb_user` compatibility with MongoDB 2.2 (used in Debian 7).

The module `mongodb_user` could not be used on Debian 7 (released around a month ago) by installing dependencies through official system packages.

The problem is that the package `python-pymongo` that installs PyMongo has version 2.2-4+deb7u1, but the `mongodb_user` module is written for the newer version of PyMongo. It seems that the `Connection` module was renamed to `MongoClient` and maybe some features added (that are not being used). Therefore one can be used as an in-place replacement for the other.