The basic process for this is to:
1. Load the configuration using the proper loader for that version. i.e.
if you're loading V1 config, then use the V1 loader.
2. If we just loaded a version that isn't current (imagine we're
currently at V3), then we need to upgrade that config. So we first
ask the V2 loader to upgrade the V1 config to V2, then we ask the V3
loader to upgrade the V2 config to V3. We keep track of warnings and
errors throughout this process.
3. Finally, we have a current config, so we merge it into the in-process
configuration that is being loaded.
This moves out the concept of a "default VM" from the Environment class
and makes it the responsibility of the V1 configuration that at least
one VM is defined on it. This lets the configuration ultimately decide
what a "default" implementation is.
This is useful so that it can take a look at the final loaded
configuration object and possibly make some tweaks to the configuration
object. The use case this was built for was so that config V1 can verify
that there is always at least a single VM defined as a sub-VM, the
"default" VM.
Previously, all procs were assumed to just be the current version. This
is certainly not going to be true always so now the version number of
the configuration must be explicit if you're assigning a proc to the
configuration loader.
This means that the Config::Loader now only knows how to load
configuration for versions used to initialize the class. This lets
things like the tests be completely isolated from what the actual
configuration is for Vagrant. This will be immensely useful to verify
that the loader functionality works for non-trivial bits (like
upgrading) without depending on Vagrant's upgrading functionality.
Config::Loader will be the new class responsible for loading configuration
and replaces the previous dual-role "Vagrant::Config" played. While this
commit is very early-stage, once this new architecture is flushed out, it
will make loading, using, and extending configuration much easier and cleaner.
Additionally, I believe this will help post Vagrant 1.0 if multi-language
configuration is implemented.
