Big Switch: Stop watchdog on interval of 0

Corrects the behavior of the watchdog process
to exit if the user configures the polling interval
to be 0.

Also adds missing documentation to sample config.

Change-Id: I17b566867c21f42985cc4662f56d32db690f471f
Closes-Bug: #1332334
Merge "Big Switch: Stop watchdog on interval of 0"