Add authenticaion support and PullImageCommand, plux a couple of fixes.

1. Add authentication support using AuthConfig.
2. Add Push command
3. Fix how PullImageCommand check if the image has been pulled
4. Add console log to PullImageCommand to log the output generated by
   docker remote API

The support for authentication allows others to write their plugins so support their private registry if the registry doesn't use the simple username/password authentication.

This pull request should resolve the following bugs/issues in docker-build-step-plugin:
https://issues.jenkins-ci.org/browse/JENKINS-24388
https://issues.jenkins-ci.org/browse/JENKINS-26166
https://issues.jenkins-ci.org/browse/JENKINS-26167