Allow config/cequel.yml to specify setting replication strategy and settings

The rake task does not provide a method to pass in options when creating the keyspace.

Based on the config file settings used in create_keyspace https://github.com/reachlocal/cequel-migrations-rails/blob/master/lib/cequel-migrations-rails/keyspace_manager.rb#L16.

This gets it working with configs we currently used, but I suppose that it might be better to have a strategy section of the config file, with :class and :options defined. But then I wonder if durable_writes should be configurable too.

I also know I'm beyond 80 chars on one of the lines, and I'm not sure how ||= with || is best handled.

I'm welcome to advice on how this should be improved.