fix: replace environment variable GCE_METADATA_ROOT with GCE_METADATA_HOST (#433)

…_HOST

* keeps consistent naming across auth packages of all languages.

    The package will now check GCE_METADATA_HOST (the new name) first; if not present, it falls back to GCE_METADATA_ROOT (the old name), then the default value.

closes [#339](https://github.com/googleapis/google-auth-library-python/issues/339).
3 files changed