adding the ability to add admin and other users to mongodb through recip...

...e, readme updates, kitchen integration tests
adds
- user_management recipe, which will require auth on mongo config
- user resource
- user provider
- users attributes
- user_management integration tests
For users to add a list of users for a db, they have to extend this array. But, if they replace the array then they will lose the admin user. 

Maybe safer to keep this empty, do

```ruby
users = [ node[:mongodb][:admin] ]
users << node[:mongodb][:users]
users.each do |user|
  # add users
end
```

when adding the users in the recipe.