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 email@example.com