Invoke backup only if configured backup disks are connected

- Previously, backup script was getting triggered for mounting any
volume.
- Now smartly checks to make sure one of the configured backup disks
are connected before initiating backup.
- Makes sure that backup happens only once after the disks are
connected.
- This is done by changing the plist init type to watch on /Volumes
instead of 'StartOnMount'. Hence the script is now run whenever
anything changes in /Volumes (not just on disk mount, but also on
eject).