supervisor: provide a modular config file

When installed from distro package, supervisor can pick up
program-specific configuration files under /etc/supervisor/conf.d/
This commit provides a modular supervisord which can be dropped
there, omitting daemon configuration.

Signed-off-by: Luca Bruno <luca.bruno@rocket-internet.de>
docs: update supervisord documentation

Update all docs to point out how to use security_monkey with
system-wise installed supervisor.

Signed-off-by: Luca Bruno <luca.bruno@rocket-internet.de>