Honor default fs name when initializing event logger.

This is related to SPARK-1459 / PR #375. Without this fix,
FileLogger.createLogDir() may try to create the log dir on
HDFS, while createWriter() will try to open the log file on
the local file system, leading to interesting errors and
confusion.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #450 from vanzin/event-file-2 and squashes the following commits:

592cdb3 [Marcelo Vanzin] Honor default fs name when initializing event logger.