Add state handling to handler processing

- Use <plugin_dir>/config/HandlerState to manage plugin state
GetHandlerState: default to 'NotInstalled'

- Return 'NotInstalled' rather than None if GetFileContents() returns
None
Merge remote-tracking branch 'upstream/2.0' into statemachine

* upstream/2.0:
  SLES12 fix for wickedd-dhcp4
Merge pull request #31 from szarkos/statemachine

Add state handling to handler processing