Resource vcd_subscribed_catalog

[dataclouder]

Implements resource vcd_subscribed_catalog that subscribes to a published catalog (Issue #776)

Improve vcd_catalog resource and data source with fields that show publishing related data:

• href
• publisher_subscription_url
• vapp_template_list
• media_item_list

TODO:

• create data source vcd_subscribed_catalog
• create resources vcd_catalog_sync, vcd_catalog_item_sync, and vcd_media_item_sync
• create tests for resource and data source
• write docs