Use newer versions of OSes.

There's a lot of duplication between the three existing collectd containers, which can be reduced.

A new container for collectd-6 needs to be added too.

Changes:

(MERGED) 72425: [docker] Reduce the number of build steps in containers | https://gerrit.opnfv.org/gerrit/c/barometer/+/72425

(MERGED) 72589: [docker][experimental] Allow COLLECTD_TAG to be passed to container build | https://gerrit.opnfv.org/gerrit/c/barometer/+/72589

(MERGED) 72412: [docker] Add collectd-6 | https://gerrit.opnfv.org/gerrit/c/barometer/+/72412