This avoids installing dnf-plugins-core, which breaks the yum and dnf modules
when uninstalling packages using a wildcard after they have already been removed.
This should resolve issues with the yum integration tests failing after docker tests run.
- use include_vars to set appropriate packages and pip packages per distribution and version
- install an older version of Docker CE on RHEL 8 since a dependency is unavailable
- disable warnings on tasks that are ok
- skip tests for CentOS/RHEL 6