devserver: add ongoing au process number check in check_health.

This CL mainly does:
1. Add checking the number of current background au processes in
check_healthy devserver call.
2. Force kill_au_proc to kill the au process if the process's pid is
passed.

BUG=chromium:696606
TEST=Run local devserver and call check_health & kill_au_proc.

Change-Id: I4fe44407b85659bd3aab309ac8efe11b0b457f68
Reviewed-on: https://chromium-review.googlesource.com/447821
Reviewed-by: Aviv Keshet <[email protected]>
Commit-Queue: Xixuan Wu <[email protected]>
Tested-by: Xixuan Wu <[email protected]>
2 files changed