Convert catalog item resource into vApp template

[dataclouder]

Catalog item is a resource that the VDC provider uses mainly as a place holder for a vApp template.
Its usefulness is mostly as a vApp template.

Problem

The current implementation is a hybrid:

• it's called catalog_item
• we upload it as catalog item
• we retrieve it as catalog item
  However:
• the metadata is assigned to a vApp template
• when we need to create a VM, we get the vApp template through the catalog item.

One consequence of this fact is that, when searching for catalog items by filter, we need to use the vApp template data. Although they are related, and one contains the other, the creation date of the catalog item and the creation date of the vApp template usually differ by a few hundred milliseconds. And, as, mentioned above, the metadata is only assigned to the vApp template.

Proposed solutions

Add a vApp template resource and deprecate catalog item. We will deal directly with vApp templates, and ignore catalog item and its quirks.
Every time we reference a catalog item in other resources, we will deal with a vApp template instead.

cc: @lvirbalas @Didainius @vbauzysvmware