Fix soft-reboot support

The soft reboot command is taken from (in this order):
* boot action `soft_reboot` key
* device dictionary (soft_reboot_command variable)
* the default constant

Signed-off-by: Rémi Duraffort <remi.duraffort@linaro.org>
15 jobs for soft-reboot in 21 minutes and 50 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #91877
amd64
dispatcher-debian-10

00:03:17

passed #91879
amd64
dispatcher-debian-11

00:03:39

passed #91878
amd64
server-debian-10

00:03:48

passed #91880
amd64
server-debian-11

00:07:17

 
  Analyze
passed #91882
amd64
black

00:01:18

passed #91883
amd64
code_quality

00:00:35

passed #91884
amd64
coverage

00:08:24

58.91%
passed #91885
amd64
dockerfiles

00:00:34

passed #91887
amd64
pylint

00:03:13

passed #91881
amd64-dind
sast

00:02:47

passed #91886
amd64
schemas

00:01:14

 
  Build
passed #91889
amd64
debian/10

00:01:57

passed #91888
amd64
doc

00:01:09

passed #91890
amd64-dind
docker-amd64-dispatcher

00:05:36

passed #91891
amd64-dind
docker-amd64-server

00:06:04