p.conf: use simple master-command

running piuparts from git cannot easily restrict the command to be run via
authorized_keys

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s/p-m: move chdir and stderr logging to master wrapper

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: stop using master-directory

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: stop using (master's) log-file

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-m.deb: restrict slave's ssh key to only allow running piuparts-master

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: support empty master command

the command may be set in authorized_keys

Signed-off-by: Andreas Beckmann <anbe@debian.org>