Allow multiple comma-separated hosts in spark.cassandra.connection.host

The current connector, before my patch, will error out if you provide a value for connection host and that host is invalid:

scala> val conf = sc.getConf
conf: org.apache.spark.SparkConf = org.apache.spark.SparkConf@c77e17b

scala> val connector = com.datastax.spark.connector.cql.CassandraConnector(conf)
java.net.UnknownHostException: doesnotexist.mediacrossing.com: Name or service not known


That's the behavior I'd expect as a user:
If I don't provide a configuration value at all, defaulting to localhost makes sense.
if I provide an explicit configuration value, but that value is not usable, I'd expect an error, not a silent default to localhost.