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

Just looking still, not compiling but my emphasis here is try to keep the code cleaner:

```scala
val hosts = conf.get(CassandraConnectionHostProperty, InetAddress.getLocalHost.getHostAddress).
          split(",").flatMap { host =>
             try Some(InetAddress.getByName(host)) catch {
                 case NonFatal(e) => // or the exact exception if you must
                    logError(s"Unknown host '$host'", e)
                     None
             }
          }.toSet
```