[WIP - DO NOT MERGE] Convert build shell scripts to go

This PR depends on Jeff's #311 PR. In addition to that, here are pending changes that should go in this afternoon:
- Move build commands to match structure introduced by PR #304
- Vendor openshift/source-to-image in origin and add use vendored STI in sti-builder
- Update code that reads dockercfg to use upstream k8s utility code