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

When we add using the Typesafe config, we will do 
```scala
Try(immutableSeq(spark.getStringList("master"))).toOption.toSeq.flatten
```

But for now try:
```scala
import akka.japi.Util.immutableSeq
val hostsWithFailover = Try(immutableSeq(conf.get(CassandraConnectionHostProperty,
      InetAddress.getLocalHost.getHostAddress).split(","))).toOption.toSet.flatten
```