Clarify "skipping cloud sub-directory placeholder object" message in cp and rsync commands
Revert "tests pass"

This reverts commit 4a27e7dc31cd7556e0fbfc54b326920b8065f332.

Conflicts:
	third_party/boto
Add Cloud SDK awareness to gsutil.

When Cloud SDK is used:
- disable update checking (since updates should be done through Cloud SDK)
- use the Cloud SDK client id & secret (since OAuth2 credentials should be acquired via gcloud auth login)
- append Cloud SDK + version to user agent strings

Update tests to honor these expectations
Advise users to run gcloud auth login for credential errors when Cloud SDK is used