Flows are not cancelled when engine is gracefully shutdown

[vijaynalawade]

Current behavior (how does the issue manifest): Start a long running flow (add Sleep activity to simulate) and gracefully shutdown engine. In this case, running flow instance and corresponding Sleep activity is not cancelled. Expected behavior: Both flow instance and activity must be cancelled. Minimal steps to reproduce the problem (not required if feature enhancement):

Please tell us about your environment (Operating system, docker version, browser & web ui version, etc):

Flogo version (CLI & contrib/lib. If unknown, leave empty or state unknown): 0.X.X

Additional information you deem important (e.g. issue happens only occasionally):