[Local demo] Make local-data-path optional

[nikhilsimha]

We don't need to specify local-data-path if the tables have already been initialized. Currently it is mandatory.

Exception in thread "main" java.util.NoSuchElementException: None.get
        at scala.None$.get(Option.scala:347)
        at scala.None$.get(Option.scala:345)
        at ai.chronon.spark.Join$$anonfun$12.apply(Join.scala:255)
        at ai.chronon.spark.Join$$anonfun$12.apply(Join.scala:255)
        at scala.Option.getOrElse(Option.scala:121)
        at ai.chronon.spark.Join.computeJoin(Join.scala:255)
        at ai.chronon.spark.Driver$JoinBackfill$.run(Driver.scala:105)
        at ai.chronon.spark.Driver$.main(Driver.scala:560)
        at ai.chronon.spark.Driver.main(Driver.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.JavaMainApplication.start(SparkApplication.scala:52)
        at org.apache.spark.deploy.SparkSubmit.org$apache$spark$deploy$SparkSubmit$$runMain(SparkSubmit.scala:855)
        at org.apache.spark.deploy.SparkSubmit.doRunMain$1(SparkSubmit.scala:161)
        at org.apache.spark.deploy.SparkSubmit.submit(SparkSubmit.scala:184)
        at org.apache.spark.deploy.SparkSubmit.doSubmit(SparkSubmit.scala:86)
        at org.apache.spark.deploy.SparkSubmit$$anon$2.doSubmit(SparkSubmit.scala:930)
        at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:939)
        at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)