Addapt mongodb types to allow resource listing.

database is a property not a parameter

database default to test, not to fail anymore
password_hash default to false, not to fail anymore
Add a username property to mongodb_user.

This allows to have users with the same name in different databases.
Reimplement mongodb_user provider

Based on Puppet::Provider::Mongodb for mongodb 2.6.x
Reimplement mongodb_database provider based on Puppet::Provider::Mongodb
Fix specs for improved type providers
Add Mongodb 2.4.x compatibility for mongodb_user
Set version to 2.6.x for spec
Document the new mongodb_user parameter.