Simplify CQL create keyspace syntax

In order to simplify the new user experience we should make the specification of the replication strategy when creating a new keyspace optional. Additionally the NetworkTopologyStrategy should be made the default with a RF of 1 per existing datacenter.

This proposal would make the two operations equivalent:

CREATE KEYSPACE test WITH replication = {
  'class': 'NetworkTopologyStrategy',
  'ExistingDC': '1'
};

CREATE KEYSPACE test;