ZWave: Improve node Heal State information

Allows storing of the Heal State information in XML if the heal Failed
or Done.
Restores the Heal State string on startup
I'd suggest changing this line to 
record.value = networkMonitor.getNodeState(nodeId);
In the event that there's never been a heal, you get IDLE rather than NONE - it can be interpretted the same in the GUI, but by ensuring that all strings come from the same place, we ensure a consistent response and if it's ever changed, it only needs to change in one place.