Enable Manage-Service to disable or enable a heat-engine

When cloud operator wants to conduct planned maintenance activity on the servers where heat-engines are deployed and running, [1] they would expect to stop the engine gracefully such that, the current IN_PROGRESS Resources (in convergence mode) and Stacks (in non-convergence mode) in that engine should be allowed to complete the current action to reach it final state either COMPLETED or FAILED. This will make sure those resources/stacks are handled gracefully without affecting them. [2] And they also do want to allow the heat-engine to take any RPC reques either from heat-api or other heat-engine workers.

But In the current heat-engine, when the stop command is issued, engine stops all running threads gracefully and does not take care of those IN_PROGRESS Resoruces/Stacks.

So as a cloud operator , I would like to 'disable' the given heat-engine or all heat-engines in a host to stop taking new RPC requests. As part of this action, I would like to allow heat-engine to complete all IN_PROGRESS Resources/Stacks.

Like heat provides roll-back action as a fall-back option, I would like to enable the disable heat-engine, in case I want to. This will be useful, if cloud operator wants to re-enable the heat-engine for some reason, such as during re-schedule in maintenance activity.