Changes

centos-repo-install-test: new image

Prepare the repo-install-test to be extended to cover centos8 as well.
The scripts from docker-repo-install-test will be moved to osmo-ci.git
as shared code, and extended to work with centos8 too (using this new
image) in I399360ee6389591180859b52f0ef08883447b90b (osmo-ci).

Related: OS#4563
Change-Id: I50743c304728444ede8e0d40b8abef9d17c44d8d