hammer.py: provide a valid freeradius-client package for RHEL 8

Packaging for RHEL 8 fails with:

09:14:16  Traceback (most recent call last):
09:14:16    File "kea/hammer.py", line 2998, in <module>
09:14:16      main()
09:14:16    File "kea/hammer.py", line 2973, in main
09:14:16      build_cmd(args)
09:14:16    File "kea/hammer.py", line 2868, in build_cmd
09:14:16      args.ccache_dir, args.pkg_version, args.pkg_isc_version, args.repository_url, pkgs_dir)
09:14:16    File "kea/hammer.py", line 2342, in build_local
09:14:16      pkg_version, pkg_isc_version, repository_url, pkgs_dir)
09:14:16    File "kea/hammer.py", line 2284, in _build_native_pkg
09:14:16      pkg_version, pkg_isc_version, repo_url)
09:14:16    File "kea/hammer.py", line 2105, in _build_rpm
09:14:16      raise NotImplementedError('missing freeradius-client version for %s-%s' % (system, revision))
09:14:16  NotImplementedError: missing freeradius-client version for rhel-8

https://jenkins.aws.isc.org/job/kea-dev/job/pkg/775/execution/node/68/log/