Improve run_tests.sh baseurl_release detection
Improve run_tests.sh baseurl_release detection

With the introduction of the centos8-stream mock configuration, we
want to allow a CI job to specify "target=centos8-stream". To do
so, we need to modify the run_tests.sh script, so it detects both
centos8 and centos8-stream as valid targets, and sets the right
baseurl and log directory in each case.

Change-Id: I633ac83b13be49f821e527a2bc790672c4b5802b

openstack-packages/DLRN,openstack-packages/delorean,openstack-packages/DLRN,openstack-packages/delorean