Control syslog-ng via ENV variables

[guedressel]

Currently syslog-ng just streams everything to a remote service.
There are some use cases where this is not convenient.

Some example switches would be

• stream to stdout
• stream to file
• allow mixing of (above) stream targets

This could be accomplished in the check.sh script...

[guedressel]

Funny fact: I implemented such a switching since the beginning but eventually forgot about it. After I read my own manual i remembered...

Though:
I refactored the logic of how the switch happens. Before every sub-image where required to switch their extra syslog-ng configs themself. But now they may put their configurations in different directories to let the base image logic use the configs from the right directory:

• /etc/syslog-ng/conf.d/*.conf
  This files will be included every time
• /etc/syslog-ng/conf-stdout.d/*.conf
  Such files are used if LOG_TO=stdout
• /etc/syslog-ng/conf-fluent.d/*.conf
  Used when LOG_TO=fluent

What's left to do ist to document these recent changes in the README.md