Wait for the NN to exit safe mode before starting services that depend on it.

Our testdata/run-all.sh can be brittle depending on the state of your Hdfs.
In particular, Yarn depends on the NN not being in safe mode, but it may take
some time for the NN to exit safe mode immediately after starting Hdfs.
This patch makes the NN startup script complete only after the NN has exited
safe mode.

Change-Id: I8b30cd07128dc48d79d91726eafed4174fb91a6d
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3005
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins