dashboard/buildlet: start of the buildlet

This is the basics: untar a tar.gz file to a directory, and execute a
command.

Update golang/go#8639
Update golang/go#8640
Update golang/go#8642

Change-Id: I5917ed8bd0e4c2fdb4b3fab34ca929caca95cc8a
Reviewed-on: https://go-review.googlesource.com/2180
Reviewed-by: Andrew Gerrand <adg@golang.org>