[feat/cli] keyspace options in configuration and un-nesting

[thibaultcha]

Reopening of #589 against feat/commented-config this time.

Possibility to configure the replication strategy used by the created
keyspace and its options.

Added options:

######
## Keyspace options. Set those before running Kong or any migration.
## Those settings will be used to create a keyspace with the desired options
## when first running the migrations.
## See http://docs.datastax.com/en/cql/3.1/cql/cql_reference/create_keyspace_r.html

######
## The name of the replica placement strategy class for the new keyspace.
## Can be "SimpleStrategy" or "NetworkTopologyStrategy".
# replication_strategy: SimpleStrategy

######
## For SimpleStrategy only.
## The number of replicas of data on multiple nodes.
# replication_factor: 1

######
## For NetworkTopologyStrategy only.
## The number of replicas of data on multiple nodes in each data center.
# data_centers:
#   dc1: 2
#   dc2: 3

Implements #543 and #350