[autotest] Run set_power_state call in a new process

Move set_power_state call in a new process, so we can attempt to timeout the
call.

BUG=chromium:243567
DEPLOY=rpm

TEST=local rpm test
tested with dut chromeos1-rack3-host2, steps:
Edit shadow config:
[CROS]
rpm_frontend_uri: http://dshi.mtv.corp.google.com:9999
Edit rpm_config.ini
frontend_addr: dshi.mtv
Start dispatcher and frontend
ssh admn@chromeos1-rack3-rpm1.cros
enter status to check rpm status
lock one machine not stumpy
./rpm_client.py -m chromeos1-rack3-host2 -s ON
check status is ON
./rpm_client.py -m chromeos1-rack3-host2 -s OFF
check status is OFF
unlock dut

Change-Id: I64b3b7f75a56a6fe4d6a2140c7fee8194bda5c7c
Reviewed-on: https://chromium-review.googlesource.com/189359
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Simran Basi <sbasi@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
3 files changed