This is a cleaner method of configuring the desired envs to run,
by overriding tox's config.envlist.

There are a few things worth noting:

1. Because we override envlist directly, instead of proxying
   through the TOXENV environment variable, it eliminates an
   inconsistency where all envs run when no environments are
   detected to run, because TOXENV was set to the empty string
   and ignored.
2. Because we override envlist later, we must also check manually
   for envs passed directly into tox, so we don't override in
   that case (config.option.env).
3. To maintain backward compatibility, envs that don't exist
   in the tox.ini aren't automatically created like they are
   when using the TOXENV environment variable, so we have to
   manually add the envconfigs necessary for those.
4. All references to TOXENV are abandoned.