Add stable branch tests for centos-8s tempest tests

Add a variation of the telemetry-dsvm-integration-centos-8s jobs using
branch selectors and provide a config for python 3.6 on older branches
(i.e. pre-yoga)

Change-Id: Ia57b83c7d764e2bb4eda19a46034ff2d3353aff7
1 file changed