Cluster test: catch FLUSHALL errors on node reset.

FLUSHALL will fail on read-only slaves, but there the command is not
needed in order to reset the instance with CLUSTER RESET so errors can
be ignored.
Cluster: always allow ok -> fail switch in clusterUpdateState().

There is a time defined by REDIS_CLUSTER_WRITABLE_DELAY where fail -> ok
switch is not possible after startup as a master for some time, however
the contrary (ok -> fail) should always be possible.