EnvironmentsConfig should be serializable

[Gustavo Niemeyer]

We need a method like:

    data = EnvironmentsConfig(path).serialize_with_environment("environment-name")

This will generate a valid "environments.yaml" configuration, containing only the environment with the given name.

Internally, this option should call the environment's serialize() method, which in turn should call the serialize() method in the machine provider. This enables the machine provider to inject additional required options such as AWS keys (which may be retrieved from the env variables) and translate the authorized_keys_path option into an authorized_keys (this depends on bug #614447 being fixed).

This is needed by the ProvisioningAgent