Update azkaban-executor-start.sh

Fixes a couple of typos that stopped script from working:

- looks like AzkabanExecutorServer lives in azkaban.execapp not azkaban.webapp

- Setting env variable, AZKABAN_OPTS, needed quotes around it.
Merge pull request #24 from aishfenton/master

Fixes a couple of typos that stopped startup script for exec service from working: