[autotest] Add some scripts into contrib.

This is my dump of useful scripts.
  * stage_build stages a build
  * dhcp_failed_machines can help you debug DHCP issues
  * watch_expensive_rpcs can help you debug cautotest RPC load

TEST=ran them
BUG=None

Change-Id: I49947a87aa2c15a2778e580075a7eb69d65f19ce
Reviewed-on: https://chromium-review.googlesource.com/176319
Commit-Queue: Alex Miller <milleral@chromium.org>
Tested-by: Alex Miller <milleral@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Prashanth B <beeps@chromium.org>
4 files changed