Health state improvements

* Display health instead of power state on the node overview page
* Consider a node as healthy if it is not in the 'error' or 'deploy
  failed' states.
* Add filtering by health state - clicking the link of nodes with
  errors only display those nodes (it used to show all nodes)

Change-Id: Ic507837496b0c660c7382637f2ee2833c6f8263c
Closes-bug: #1302021
Merge "Health state improvements"