Capitalize bash variable and prefix with KUBE_

We intend to make these setable by the environment.  This just does the
renaming.
Allow setting of git_commit, version, and dirty via shell varables

This is particularly useful if building from a tarball instead of from a
checked out git repo (as all Linux distributions do)
Merge pull request #1174 from eparis/version-shell-overrides

Allow setting of git_commit, version, and dirty via shell varables