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
Change LastHeal to use NodeState

Returns IDLE instead of NONE when no previous Heal information.
Simplifies code
Fix LastHeal state

Fixing as per Chris' suggestions
Merge pull request #1523 from wezhunter/zwave-improve-healstate

ZWave: Improve node Heal State information