Migrate Cassandra keyspace to NetworkTopologyStrategy

[subnetmarco]

We should change the replication strategy of the Kong keyspace to NetworkTopologyStrategy as opposed to having SimpleStrategy to make it easier to scale a Cassandra cluster. From the Cassandra documentation:

NetworkTopologyStrategy: Highly recommended for most deployments because it is much easier to expand to multiple data centers when required by future expansion.

It means that NetworkTopologyStrategy can work pretty much in every scenario (including single-machine, or single-datacenter), while SimpleStrategy is more limited.

Doing this should be as easy as executing:

ALTER KEYSPACE kong WITH REPLICATION = {'class' : 'NetworkTopologyStrategy', 'DC1' : 1};

This also brings another question: in a production environment I might be wanting to setup my own Cassandra cluster with some very specific settings, how do we prevent Kong migrations from being executed and reverting these settings?

[subnetmarco]

This seems to work when invoked by cqlsh, but the same command fails when executed from the migration script:

[ERR] Cannot record migration 2015-06-09-170921_0.4.0: Cassandra error: Cassandra returned error (Unavailable exception): "Cannot achieve consistency level ONE"

Very weird since the same command seems to be working manually:

ALTER KEYSPACE kong WITH REPLICATION = {'class' : 'NetworkTopologyStrategy', 'DC1' : 1};

[thibaultcha]

See #543 for what needs to be done here.

[thibaultcha]

Implemented with #634.