Fix NetconfNodeHandler interactions

Fix NetconfNodeHandler interactions

There are a couple of failures in interacting with RemoteDeviceHandler
API (which is completely undocumented).

The API design (and interactions in implementations) assume that:
- onDeviceConnected()/onDeviceDisconnected() manage a reconnecting
  process -- i.e. when 'disconnected' it is assumed the device might
  come back again
- onDeviceFailed() is the hard alternative, indicating the node is node
  coming back

This is then picked up and routed through NetconfTopologyDeviceSalFacade
to NetconfDeviceTopologyAdapter, which makes assumptions and propagates
them to the datastore.

JIRA: NETCONF-1097
Change-Id: Ib148a617b52e775261212aa3a809d1ab450d7d13
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>