Resolve scheme-less event log directory relative to default filesystem

Currently, if I specify a default filesystem:

fs.defaultFS=hdfs://host

And a scheme-less event log directory:

spark.eventLog.dir=/some/dir

Then the event log directory is always resolved with respect to the local filesystem.

I propose that it should be resolved with respect to the default filesystem.

Targeting Spark 3.0.0 since this is technically a breaking change.