The secretAccessKeysRevoke revoke function now asserts that it is
not dealing with STS keys by checking a new internal data flag. Defaults
to IAM when the flag is not found.
Factored out genUsername into its own function to share between STS and
IAM secret creation functions.
Fixed bad call to "WriteOperation" instead of "UpdateOperation" in
aws/backend_test
The new STS path allows for obtaining the same credentials that you would get
from the AWS "creds" path, except it will also provide a security token, and
will not have an annoyingly long propagation time before returning to the user.
