lava-dispatcher: add support for pre power/os command in minimal boot action

Once boot order set to boot from USB if available on devices like
raspberry pi that support the boot order, USB drive can be used as
primary media to deploy test image to mmc/emmc.

Run `pre_os_command` to power off USB drive before applying power will
boot the device from mmc/emmc with new deployed test image.

Signed-off-by: Chase Qi <chase.qi@linaro.org>
18 jobs for pre-mini-boot in 17 minutes and 11 seconds (queued for 10 seconds)
Status Name Job ID Coverage
  Test
passed dispatcher-debian-10 #140328
amd64

00:03:08

passed dispatcher-debian-11 #140330
amd64

00:03:08

passed server-debian-10 #140329
amd64

00:03:44

passed server-debian-11 #140331
amd64

00:05:52

 
  Analyze
passed bandit-sast #140332
amd64-dind

00:01:00

passed black #140335
amd64

00:00:49

passed code_quality #140336
amd64

00:00:29

passed codespell #140341
amd64

00:00:26

passed coverage #140337
amd64

00:07:00

61.11%
passed dockerfiles #140338
amd64

00:00:25

passed eslint-sast #140333
amd64-dind

00:00:31

passed pylint #140340
amd64

00:02:41

passed schemas #140339
amd64

00:00:35

passed semgrep-sast #140334
amd64-dind

00:01:15

 
  Build
passed debian/10 #140343
amd64

00:01:28

passed doc #140342
amd64

00:00:36

passed docker-amd64-dispatcher #140344
amd64-dind

00:03:47

passed docker-amd64-server #140345
amd64-dind

00:04:14