Add a service provider

[ls339]

This PR will allow you to specify a service provider in your manifests.

For example:

node "ip-127-0-0-1" {
    class { "datadog_agent":
        api_key => "xxxxxxxxxxxxxxxxxxxxxxxxxxx",
        service_provider => 'upstart',
    }
}

[truthbk]

Quite close to what we want, I will cherry-pick some of this when #444 is merged so we can add the option to the main manifest for configuration simplicity.

[truthbk]

Big Thanks @ls339 - superseded by #444, closing!