Glance metadefs tags support in Horizon

Summary
=======
Support for tagging is already available in the glance v2 API.
Horizon needs to add tagging support for images and instances to expose this functionality.

Motivation
========
Glance v2 support tagging. The motivation is being able to assign tags to images and instances.

Description
=========
1. Retrieve and expose the list of available tags.
2. Modify metadata widget to allow adding and removing tags.

UX
===
New panel in metadata widget displaying tags as bootstrap labels in input-like field.
Input at the end of the field allowing to add new tags.

Wireframes, Mocks, Videos and UI Markup
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http://jsfiddle.net/gkypoxbm/

Testing
======
Open modal with metadata widget, add couple of tags, save.
Reopen, remove some of the tags.
Reopen, confirm that only remaining tags are displayed.

Outside Dependencies
==================
https://blueprints.launchpad.net/glance/+spec/metadefs-tags

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

Doc Impact
=========
N/A