dashboard/env: update watcher Dockerfile to use latest version

This version includes the JSON polling from:
   https://go-review.googlesource.com/#/c/1568/

Also, we no longer need sid because we no longer need Mercurial,
so revert back to wheezy, which is less of a moving target.

Change-Id: I3e6bde097e49deb891101cd92fa80e9f093a7c8b
Reviewed-on: https://go-review.googlesource.com/1613
Reviewed-by: David Symonds <dsymonds@golang.org>
dashboard/coordinator: add SSD disk option, turn off auto-update

Change-Id: I4d2dfe34a6c2fde8603704e716b7c91ad285ecec
Reviewed-on: https://go-review.googlesource.com/1561
Reviewed-by: Andrew Gerrand <adg@golang.org>