Check for existing entry before adding to crontab

Closes #18 

To test:
0. Begin with a configured 1.0/1.1 node. Log in via ssh and make sure the commotion-dashboard-helper entry appears only once in crontab (`crontab -l`). Delete any duplicates.
1. Run sysupgrade using this branch.
2. Check crontab again. There should not be a duplicate entry for commotion-dashboard-helper.