Make credentialprovider less verbose about benign errors.

In particular, a few of the utilities used within the credentialprovider had the pattern:
   glog.Errorf("while blah %s: %v", s, err)
   return nil, err

This change composes the error message that is propagated and puts the burden of logging on the caller, which gives us the new pattern:
   return nil, fmt.Errorf("while blah %s: %v", s, err)

In particular, this allows us to squelch all output during kubelet startup when we are detecting whether certain credentialprovider plugins should even be enabled.

Fixes: https://github.com/GoogleCloudPlatform/kubernetes/issues/2673