Parsing error with comma-separated `--files` parameters

[ash211]

Seen during testing an internal application:

 Exception in thread "main" org.apache.spark.SparkException: File file:/path/to/logback.xml,file:/path/to/truststore.jks does not exist or is a directory.
     at org.apache.spark.deploy.kubernetes.Client$$anonfun$run$2.apply(Client.scala:84)
     at org.apache.spark.deploy.kubernetes.Client$$anonfun$run$2.apply(Client.scala:82)
     at scala.collection.immutable.List.foreach(List.scala:381)
     at org.apache.spark.deploy.kubernetes.Client.run(Client.scala:82)
     at org.apache.spark.deploy.kubernetes.Client$.main(Client.scala:810)
     at org.apache.spark.deploy.kubernetes.Client.main(Client.scala)
     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
     at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
     at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
     at java.lang.reflect.Method.invoke(Method.java:498)
     at org.apache.spark.deploy.SparkSubmit$.org$apache$spark$deploy$SparkSubmit$$runMain(SparkSubmit.scala:747)
     at org.apache.spark.deploy.SparkSubmit$.doRunMain$1(SparkSubmit.scala:178)
     at org.apache.spark.deploy.SparkSubmit$.submit(SparkSubmit.scala:203)
     at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:117)
     at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)

cc @mccheah