Modify dhcp_failed_machines for nagios nrpe

Add two more optional arguments (warning and critical threshold), so nagios
nrpe can directly call the script to check dhcp status.

BUG=chromium:333107
TEST=manual test in chromeos-mcp1 and chromeos-sam1

Change-Id: I35c85fa9d5209846c018d112d28139200e3e2ffb
Reviewed-on: https://chromium-review.googlesource.com/182853
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Alex Miller <milleral@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
1 file changed