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.
I created VersionBase which is the abstract base class for any
configuration versions. Configuration versions are responsible for
knowing how to load configuration given a proc (from a
Vagrant.configure block), as well as merging configuration procs. In the
future, it will have to upgrade versions as well. This is not done yet.
The VERSIONS constant was added to Vagrant::Config which is a registry
to keep track of all the available configuration versions. The
VERSIONS_ORDER constant is an array of the ordering of these versions.
The ordering is important so that in the future Vagrant can attempt to
gracefully upgrade the configurations. It is also used to determine the
current configuration version (which is assumed to be the last version
in the order).
The loader was modified to use the current version and the VERSIONS
registry instead of hardcoding V1.
