[SCT] Get rid of CentOS distro across tests and manager-related framework modules

[mikliapko]

Even though all jobs have been switched from utilizing CentOS distro for manager-server node, we still have several places where CentOS is mentioned in names of variables, methods, etc.

Just a couple of examples:

• https://github.com/scylladb/scylla-cluster-tests/blob/master/sdcm/mgmt/cli.py#L924
• https://github.com/scylladb/scylla-cluster-tests/blob/master/sdcm/mgmt/common.py#L24
• https://github.com/scylladb/scylla-cluster-tests/blob/master/defaults/manager_versions.yaml#L3

That's not the full list, an additional research across the framework should be done in scope of this task as well.