Override reference.conf to Support gs FileSystem

I am using 0.11 version of MLeap and Trying to add support for GS

I checked bundle-hdfs/src/main/resources/reference.conf and found that we can use other schemes like S3 to store the MLeap Bundle in S3. I wanted to add support for Google Storage (GS) scheme.

I created new reference.conf in the server and added it to spark classpath using `--conf spark.executor.extraClassPath="/home/myuser/reference.conf"`. I get below error:

```
Exception in thread "main" java.util.NoSuchElementException: key not found: gs
at scala.collection.MapLike$class.default(MapLike.scala:228)
at scala.collection.AbstractMap.default(Map.scala:59)
at scala.collection.MapLike$class.apply(MapLike.scala:141)
at scala.collection.AbstractMap.apply(Map.scala:59)
at ml.combust.bundle.BundleRegistry.fileSystemForUri(BundleRegistry.scala:146)
at ml.combust.bundle.BundleWriter$$anonfun$save$5.apply(BundleWriter.scala:51)
at ml.combust.bundle.BundleWriter$$anonfun$save$5.apply(BundleWriter.scala:50)
at scala.util.Success$$anonfun$map$1.apply(Try.scala:237)
at scala.util.Try$.apply(Try.scala:192)
at scala.util.Success.map(Try.scala:237)
at ml.combust.bundle.BundleWriter.save(BundleWriter.scala:49)
```
NOTE : I changed the `HadoopBundleFileSystem` which can default to `Seq("hdfs", "gs")` and it worked. I need help in finding a way to propagate the settings via reference.conf with no code change. Any help is appreciated.