split s3Push() -> s3Push{Venv,Docker}() methods
split s3Push() -> s3Push{Venv,Docker}() methods

The original virtualenv/pip implementation is retrained (with slight
refactoring) as s3PushVenv(), which is continued to be used on OSX.
Linux tarball builds will use s3PushDocker(), which currently uses the
docker.io/lsstsqre/awscli image.

jhoblitt/jenkins-dm-jobs,lsst-sqre/jenkins-dm-jobs