Add vApp Template support for vcd_vapp_vm and vcd_vm resource and data sources

[adambarreiro]

Description

This PR:

• Builds on top of Add placement policy to vcd_vapp_vm and vcd_vm resources and data sources #922
• Requires Add Media item retrieval functions go-vcloud-director#520

The proposed changes add the following attributes to vcd_vapp_vm and vcd_vm resource and data source:

• vapp_template_id: The URN of the vApp Template to use. Deprecates template_name. The idea is to use the newest vcd_catalog_vapp_template data source to fetch this unique URN.
• boot_image_id: The URN of the media to use as boot image. Deprecates boot_image. The idea is to use vcd_catalog_media datasource to fetch this unique URN.

As the URNs are unique, the need of catalog_name is no more, so it has been deprecated as well.

This PR deprecates vcd_catalog_item as well.

Not included

This PR doesn't change resource vcd_inserted_media, which will remain in backlog as it requires to use IDs instead of names.

[lvirbalas]

This is important change. Let's write it up in a blog post too, @adambarreiro

[janbaer]

How can I migrate my TF state to the new version without destroying and recreating all of my VMs? This breaking change is really dangerous.

[adambarreiro]

How can I migrate my TF state to the new version without destroying and recreating all of my VMs? This breaking change is really dangerous.

Thanks for your feedback. I'd say that in this case (existing VMs) it's better to stay with current template_name attribute, while using vapp_template_id for the new ones. The template_name didn't change its behavior and still works, the Deprecated warning is more of a heads up to switch to vapp_template_id whenever possible.
We will try to find a proper solution for this scenario that guarantees that VMs don't get recreated.