Instead of requiring the user to run the test suite with and without the
--use-old-replica-state flag, we introduce an 'old_replica_state()'
parametrized fixture that is used only when needed (i.e. in
test_cluster_{has_replica,node_count}.py).
* README: add information pertaining to shell completion;
* CONTRIBUTING: remove release information;
* RELEASE: create a dedicated file with all the relevant release
information.