Add ability to use a config file

These commits focus entirely on enabling the user to use a config file, and offer more fine grained control over those aspects of their mongo instance.

Simply provide a role like this in your Vagrantfile/roles
```ruby
:mongodb =>  
{
  :dbpath => "/for/the/luls",
  :package_version => "2.4.4",
  :use_configfile => true
}
```
or override 
```ruby
# in your new recipe
node.override[:mongodb][:use_configfile] = true 
```

Also, you can set the config file name, and location like so:

```ruby
# in your recipe, or override in a role
node.override[:mongodb][:configfile] = "/etc/non_standard_mongo.conf"
```