GCE_METADATA_ROOT should be called GCE_METADATA_HOST to be aligned with other client libraries #339
Description
file https://github.com/googleapis/google-auth-library-python/blob/master/google/auth/environment_vars.py
contains GCE_METADATA_ROOT added in:
b33b802
Client libs for other languages call it GCE_METADATA_HOST:
go: https://github.com/googleapis/google-cloud-go/blob/master/compute/metadata/metadata.go
java: https://github.com/googleapis/google-auth-library-java/blob/master/oauth2_http/java/com/google/auth/oauth2/ComputeEngineCredentials.java
...
Maybe consider supporting both names for a transition?