Switch to multi-platform release build

Update build-release to match latest state of openshift/origin. It now uses a docker image to create releases for mac(darwin), win, and linux(amd64)
hmm, so the Dockerfile for the image has a WORKDIR that is based on the project's name:
```
WORKDIR /go/src/github.com/openshift/origin
```
but I guess we could make it more generic. 

@smarterclayton would you be ok with separating the buildenv image into its own repository and pushing it to dockerhub?