devserver: refactor check_health to a separate app

This CL move all /check_health related code to a separate module and
wrap it as a standalone cherrypy application.

BUG=chromium:993621
TEST=1. Ran devserver_integrated_test.py in chroot
 2. Ran below command locally to verify:
 $ curl  http://127.0.0.1:8080/check_health

Change-Id: Ic9c483202a2cef54d6953924e90001c9937a4fa8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/1753193
Tested-by: Congbin Guo <guocb@chromium.org>
Auto-Submit: Congbin Guo <guocb@chromium.org>
Reviewed-by: C Shapiro <shapiroc@chromium.org>
Commit-Queue: Congbin Guo <guocb@chromium.org>
2 files changed