Use CASSANDRA environment variables in tests

This allows us to do testing using a Cassandra server on another host
besides localhost:9160. This will be useful for using Docker pointed at
another container.
Set defaults for os.environ.gets in test
Make sure cassandra_port is an int

Otherwise things panic by being dumb
Move cassandra_host settings to unitgration tests
Merge pull request #338 from rackerlabs/test_alternate_cassandra

Use CASSANDRA environment variables in tests