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.

TEST=1. Ran in chroot
 2. Ran below command locally to verify:
 $ curl

Change-Id: Ic9c483202a2cef54d6953924e90001c9937a4fa8
Tested-by: Congbin Guo <>
Auto-Submit: Congbin Guo <>
Reviewed-by: C Shapiro <>
Commit-Queue: Congbin Guo <>
2 files changed