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 propagates those error message and puts the burden of logging on the caller.

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
clean up error message on labels validation errs
Add namespacing for label keys
Loosen DNS 952 for labels
Add a test for important URLs.
Relax search criteria to find both pod & bound pod events
Document API doc path.
Fix a logic error in the events e2e test.
Adding an e2e test for monitoring.
Update exec health check message logged

Update message logged after performing exec health check per #2708.
Remove empty lines when file is generated from jinja template
Merge pull request #2709 from mindscratch/master

Update exec health check message logged
Merge pull request #2588 from lavalamp/fix4

Relax search criteria to find both pod & bound pod events
Merge pull request #2713 from komljen/master

Remove empty lines when file is generated from jinja template
Proper appending of logging.write scope to array
Merge pull request #2705 from brendandburns/fix

Fix a logic error in the events e2e test.
Add a test for important URLs on the apiserver.
Fix e2e failure of events test.
Add some backward compatability for GKE.
Merge pull request #2717 from erictune/event_e2e_flake

Fix e2e failure of events test.