Move back adduser call from postinst.py to lava-server.install

The LAVA system user creation in postinst.py script isn't distro-agnostic.
It's assuming using the Debian perl wrapper around useradd.
Leave the user creation to Debian specific packaging to make postinst.py
distro-agnostic and allow the user to be created separately.
Often, on other Linux distributions, adduser is a symlink to useradd,
useradd being the only portable way to create a user.

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
15 jobs from adduser in 11 minutes and 7 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #33190
amd64
dispatcher-debian-10

00:03:09

passed #33188
amd64
dispatcher-debian-9

00:02:58

passed #33191
amd64
server-debian-10

00:02:25

passed #33189
amd64
server-debian-9

00:02:52

 
  Analyze
passed #33192
amd64
black

00:00:34

passed #33194
amd64
code_quality

00:00:23

passed #33193
amd64
codestyle

00:00:38

passed #33197
amd64
job-schema

00:00:47

passed #33195
amd64
pylint

00:02:16

passed #33196
dind amd64 allowed to fail
sast

00:03:35

 
  Build
passed #33200
amd64
amd64/pkg-debian-10

00:01:33

passed #33199
amd64
amd64/pkg-debian-9

00:01:40

passed #33198
amd64
doc

00:01:15

 
  Docker
passed #33201
dind amd64
amd64/lava-dispatcher

00:01:16

passed #33202
dind amd64
amd64/lava-server

00:02:39