Refactor ShutdownProvider.shutdown()

Refactor ShutdownProvider.shutdown()

Returning a Boolean is confusing with regard to error reporting:
- users are really ignoring the value and rely on exception cause
- that is not what the code does, as it is confused about mapping,
  running set(true/false) followed by an optional setException()

Change the future type to Void, making it the value does not hold
anything and rely on setException() to deliver the failure cause.

Change-Id: Id6d97580b8d908caf37338f3578045598d2555f1
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>