Sanity around plugins in the big tent

Summary
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To clearly document and enable Horizon's policy regarding plugins.

Motivation
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With the growth in OpenStack, plugin guidelines need to be more clearly documented. There are more projects in the big tent than Horizon can support in tree. Additionally, there is more code in tree than Horizon can really support with the existing core structure. We need to document the policy for what items are in tree, what are p

Description
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There are several components of this blueprint.

Some content already in the horizon tree should be moved to openstack_dashboard/contrib. This folder is to provide some isolation for projects where reviews will come largely from the service teams. Content for Sahara and Trove will be moved. Potentially Heat as well. In addition to isolating this code, moving will illuminate some of the components missing in the existing plugin mechanism.

Adding support for adding additional policy files with a plugin will be added.

Additional work will be needed to figure out how to support integration tests from plugins.

Based on the above work and principles decided upon the resulting policy and standards will be documented and a topic added in the developer docs.

UX
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The end user experience should be unaffected.

Wireframes, Mocks, Videos and UI Markup
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N/A

Testing
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Most of this will be documentation. However, some existing content will move locations. Making sure this content is still working will be the chief test.

Documenting plugin testing best practices will be included.

Outside Dependencies
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N/A

Requirements Update Required
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N/A

Doc Impact
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This patch is mostly documentation. New documentation detailing plugin policy in Horizon will be added.