Generate unique working directory for each collector backend

[mpfz0r]

Each collector relies on a place to write its own logs, spool files, etc.
We want to ensure that multiple collectors don't step on each others toes.

Possible solution:

• Redefine collector_configuration_directory: to collector_data_root_directory:
  with a default of /var/lib/graylog-sidecar/collector-data/

• For each configuration assignment, auto-generate the following subdirectories:
  $collector_data_root_directory/<backend-name>/<config-name>/ or maybe
  $collector_data_root_directory/<backend-id>/<config-id>/

• Report the value of collector_data_root_directory back to Graylog, so we can
  generate a variable ${sidecar.dataRoot} and maybe ${sidecar.collectorDataDir}.
  This can be used in collector configurations like:

path.data: ${sidecar.collectorDataDir}/data
path.log: ${sidecar.collectorDataDir}/log

Open questions:

• Which permissions should we give this directory?
• Do we need clean up on de configuration of a collector
• Do we need a UI

[bernd]

We do this now. Yay!

[mpfz0r]

Hah. Totally forgot about my own ticket :-D