ALTER SYSTEM / nodetool enhancements

CASSANDRA-7370 mentions the idea to enhance nodetool to do something like nodetool> set config foo='bar'. This could also be done via a "special" CQL ALTER SYSTEM SET foo='bar';.

In general this ticket is just meant not keep in mind that there should be some option to change configuration using a generic approach. Either implementation (nodetool via JMX or CQL statement) should internally end at the same code.