[SPARK-11020][CORE] Wait for HDFS to leave safe mode before initializing HS.

Large HDFS clusters may take a while to leave safe mode when starting; this change
makes the HS wait for that before doing checks about its configuraton. This means
the HS won't stop right away if HDFS is in safe mode and the configuration is not
correct, but that should be a very uncommon situation.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #9043 from vanzin/SPARK-11020.