allow customizing cluster options using an init file

[dolsem]

Currently cassandra-migrate does not provide a way to customize cluster options, such as policies for load balancing, address translation (crucial when the cluster is in a private subnet and you need to map internal ips to external hostnames), etc.

This PR adds a way to override cluster options by defining an init file, e.g. cassandra-migrate.init.py, with contents like:

from cassandra.cluster import Cluster
from cassandra.policies import WhiteListRoundRobinPolicy

def create_cluster(**kwargs):
  kwargs['load_balancing_policy'] = WhiteListRoundRobinPolicy(['127.0.0.1'])
  return Cluster(**kwargs)

I'd like to know if there is interest in merging this PR. If there is, I will add tests and documentation.