This commit enhances django workflow implementation to allow horizon
plugins to add workflow steps to a workflow in other repository like
the main horizon repo. New setting "EXTRA_STEPS" is introduced to
the horizon plugin 'enabled' file.
To this aim, the workflow class looks up HORIZON_CONFIG['extra_steps']
with its class full name and loads them as extra steps if any.
HORIZON_CONFIG['extra_steps'] are populated via horizon plugin settings.
This commit completes the blueprint.
blueprint horizon-plugin-tab-for-info-and-quotas
Change-Id: I347d113f47587932e4f583d3152e781ad1a4849f
e50a69d69f