logging: reduce the Scope required to use the logging client.
logging: reduce the Scope required to use the logging client.

cloud-platform scope allows reading/managing of data across all
GCP services. logging.write allows submitting log data to your
projects. By the principal of least privilege we should prefer
the later if possible.

Change-Id: I7d9e0db90c1b2df216f4a95cad3cd01deeeb507f
Reviewed-on: https://code-review.googlesource.com/3910
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go