[data processing] Add capability to edit templates

Summary
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Currently, the data processing system only allows create/copy for templates. This blueprint seeks to add "edit" as an option for both node group templates as well as cluster templates.

Motivation
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Forcing a user to copy a template (and use a new name...and then subsequently update any templates that depend upon it) is suboptimal. We should allow editing of existing templates.

Description
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As discussed at the Kilo summit, the "edit" action will be added to each row of both the node group templates and cluster templates tables.

UX
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This was discussed at the Kilo summit. Edit is an expected feature. Its absence has been noted by just about everyone who has used Sahara.

Wireframes, Mocks, Videos and UI Markup
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N/A

Testing
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Tests will be added to verify editing of both node group templates and cluster templates.

Outside Dependencies
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There are changes required to the Sahara service in order to suppor the update method that will be used to edit the templates. The client library already has support for the update method, but the Sahara service just returns "not implemented" at the moment.

Here are the blueprints for those work items. They both need to be finished before
the Horizon change can be merged.

Sahara service: <tbd>

Requirements Update Required
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N/A

Doc Impact
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N/A