cmd/gomote, buildenv, buildlet: move config code to common places

Split off from Quentin's https://golang.org/cl/29399

Change-Id: I4578f8f485e97d6b9844fb12e84779167755752e
Reviewed-on: https://go-review.googlesource.com/29858
Reviewed-by: Quentin Smith <quentin@golang.org>
7 files changed