[TripleO] tripleo-ci-centos-8-containers-multinode now using ephemeral Heat for master/wallaby

For awareness, I wanted to point out that tripleo-ci-centos-8-containers-multinode is now using ephemeral Heat on the master and wallaby branches. This change[1] merged yesterday that made the switch. Of note, the Heat logs for the overcloud deployment are now under the directory undercloud/home/zuul/overcloud-deploy/overcloud/heat-launcher/log in the job logs.

There is also a pending docs patch that covers a bit more how the feature works: https://review.opendev.org/c/openstack/tripleo-docs/+/783008

[1] https://review.opendev.org/c/openstack/tripleo-quickstart/+/777108