Horizon Update Feature

Summary
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There should be support for a single click or guided(update wizard) featured option for upgrading the Horizon to the current stable release. It will be helpful for user, if we can provide GUI options to upgrade or update the horizon to the current stable release.

Motivation
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In all the other CMS and other Software Packages, we find an option to upgrade the package by just following some upgrade wizard clicks. If we can get the same feature for horizon, it will make the upgrading of horizon convenient for the users.

Description
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We should have an upgrade option or functionality built into the horizon itself, to upgrade the system to the current stable release of the system. In the current scenarios, we have to follow the upgrade instructions manually, which is very troublesome and requires a lot of efforts. If we can provide the upgrade option for horizon, same as we find in other CMS or other packages, it will make the life the of system admin easier. There would be functioning, by which we can check for updates if updates are available. Then, if we find any update, then we can update our system by following the update wizard of the system.

UX
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- Add "Updates" Dashboard to horizon.
- Add "check updates" button to check for any available updates.
- Add "Update" button to update the system to the current stable release, if any available.

Testing
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Proper Tests would be added to check the system's behavior after adding this feature.

Outside Dependencies
==================
NA

Requirements Update Required
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Update horizon requirement version for the new feature.

Doc Impact
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Horizon documentation will be updated in order to expose to the user the new features that will be introduced. The details about the configuration of enabling the new features will be provided in the Documentation. All the details about this new functionality should be added in the OpenStack Documentation.