[task-relaunch] Allow update existing task definition

[guoyiang]

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Problem description: At current design, task definition is immutable. It cannot be updated once created. It creates a problem if the user has properties in task definition and it may get changed over time. e.g. csvImport --fileServer=xxx, the file server may change.

Solution description: Allow task definition to be updated, or overridden.

Description of alternatives: Delete and recreate the task definition, which is feasible today. But this approach creates a challenge on availability of task. A request to launch task may come in between after deletion but before creation. Also failures in creating new task definition may make the downtime longer.

Additional context: This comes out as part of discussion on #3962 . The idea is to use task definition to provide deployer properties and app properties, but this will require the properties can be changed easily.

Thanks!

[cppwfs]

@guoyiang Adding this to the feature backlog. Thank you for the issue.
Also checkout 2.8 milestones. We have made improvements on updating deployment and app properties on the UI.

[markpollack]

See #3962 and #5053