Allow updating metadata on an instance

Summary
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There is currently no way to edit the metadata of an instance so this blueprint will add an Update Metadata action to the Instances table.

Motivation
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Users have the ability to edit server metadata using the nova CLI but this ability is not exposed through horizon.

Description
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A new action will be added to the Instances table (Update Metadata) to allow updating the metadata. This will be basically the same as the actions currently available for editing the metadata of things like images and flavors. The existing metadata definitions modal will be used for this purpose so the user can choose to add/edit existing metadata definitions or add custom properties.

UX
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The new action and modal will match the existing actions and modals for other tables like images and flavors.

Testing
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Tests will be similar to existing tests for images and flavors metadata editing.

Outside Dependencies
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None

Requirements Update Required
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None

Doc Impact
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None