Keysone project metadata

Summary
=======
Horizon should provide UI to create, read, update and delete projects metadata.
Implement CRUD for projects metadata.
- Server: implement REST API for fetching and updating metadata
- Client: re-use angular Metadata widget for UpdateMetadata actions

Motivation
=========
Currently there is no way to set any metadata to projects.
And a user might want to have this feature in case of using projects like
Stacklight to build reports on aggregated on different data including metadata items.
So it would be nice for user to have UI to modify metadata for some special tenant.

Description
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- User can specify multiple sets of key=value pairs, which will be the projects metadata.
- Key/values also could have non-latin encoding.
- Implementation could re-use existing angular metadata widget and all related js-code.
- Target API is keystoneclient
- Metadata is stored using Keystone extra properties of Project

UX
===
Affected views should show "Update metadata" row action for projects.
At least user should have ability to update project metadata in Identity/Projects panel.
UI should have "UpdateMetadata" row action for projects.

Wireframes, Mocks, Videos and UI Markup
---------------------------------------------------------------
N/A

Testing
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Should be covered by karma testcases.

Outside Dependencies
==================
https://review.openstack.org/#/c/296246/ this patch (or another fix for that bug)
should be merged into python-keystoneclient.
Without this fix user can't delete metadata items.

Requirements Update Required
========================
N/A

Doc Impact
=========