replace centos-7 with centos-8 in scenario jobs
replace centos-7 with centos-8 in scenario jobs

standalone / standalone scenario jobs move to centos-8

Related-Bug: #1865525
Change-Id: Id65c0d1c58db0d399d3e170b98e1df54c67ef49d

openstack/tripleo-common,openstack/tripleo-common