Change the script for testing to test Py 3 only
Change the script for testing to test Py 3 only

Also, drop testing CentOS:7 and provide CentOS 8 testing as usual.
CentOS 8 is using dnf as default package manager,
so it was decided to drop yum supporting as well.

*CLOUDBLD-3286

Signed-off-by: Salatskyi Serhii <3a2b708ccf3db09237dcdc2ae263a91d7c669197@redhat.com>

DBuildService/atomic-reactor,DBuildService/atomic-reactor,fr34k8/atomic-reactor,projectatomic/atomic-reactor,projectatomic/atomic-reactor,fr34k8/atomic-reactor