[docker] Add collectd-6

[docker] Add collectd-6

* Add instruction to docs/ for using
  docker/barometer-collectd-experimental/Dockerfile to build collectd
  version 6
* Add a new COLLECTD_FLAVOR to src/collectd/Makefile to configure
  tags for a collectd 6 build.
* Allow passing COLLECTD_CONFIG_CMD_ARGS to the collectd build
  For testing with new plugins for collectd-6, as un-ported plugins are
  disabled by default.
  This new option lets the ./configure command be run with extra args,
  e.g. --enable-cpu --enable-<my-newly-ported-plugin>, which means that
  plugin can be enabled for the PR that is being tested.
* Update flavor_image_name selection to include collectd-6

Change-Id: I55ee0bbcb5d89357b1ef11ab17c5b029603ecdf7
Signed-off-by: Emma Foley <efoley@redhat.com>