Remove obsolete checks and exception handling from s390 plug-in

The s390 plug-in has some strange checks on the distribution
version before collecting a couple of s390-specific commands. The
entire code block is also wrapped in try/except.

Remove this: all add_cmd_output() succeed without exceptions even
if the command does not exist or cannot be run. The exception
handling also does nothing useful.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
Remove legacy system support from cluster plug-in

The cluster plug-in attempted to support three prior generations
of cluster components in a single plug-in. These will never be
required today since all current versions where sos-3.x would be
used are all using much later packages.

Remove this code and simplify the overall plug-in organisation.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>