spark-submit: add exec at the end of the script

Add an 'exec' at the end of the spark-submit script, to avoid keeping a
bash process hanging around while it runs.  This makes ps look a little
bit nicer.

Author: Colin Patrick Mccabe <cmccabe@cloudera.com>

Closes #858 from cmccabe/SPARK-1907 and squashes the following commits:

7023b64 [Colin Patrick Mccabe] spark-submit: add exec at the end of the script

(cherry picked from commit 6e9fb6320bec3371bc9c010ccbc1b915f500486b)
Signed-off-by: Reynold Xin <rxin@apache.org>