Support setting a seed provider

[hseeberger]

Instead of setting fixed seed nodes Cassandra allows setting a seed provider, see http://docs.datastax.com/en/cassandra/2.2/cassandra/configuration/configCassandra_yaml.html?scroll=configCassandra_yaml__seed_provider for details.

I suggest to add a CASSANDRA_SEED_PROVIDER env variable. If that is set, ignore CASSANDRA_SEEDS and instead rewrite configuration along the lines of:

seed_provider:
    - class_name: $CASSANDRA_SEED_PROVIDER

[analytically]

👍

[tianon]

I think the discussion over on the PR starting at #39 (comment) is probably a good read (I'll refrain from copying it here). 😄 👍