Obtain list of images not declared in VM TEMPLATES

[OpenNebulaSupport]

Description
Get list of images not declared in any templates. Also possibly implement an extra field in the image XML, similar to VMS (which shows the Virtual Machines using the image) but with teh VM_TEMPLATES.

Use case
With the time passing, it is possible for an opennebula setup to be full of images whose templates are no longer existing. There is no quick way of doing so.

Interface Changes

• CLI: Something like oneimage list --orphans
• Sunstone: Templates Tab, similar to VMS tab.

Progress Status

• Branch created
• Code committed to development branch
• Testing - QA
• Documentation
• Release notes - resolved issues, compatibility, known issues
• Code committed to upstream release/hotfix branches
• Documentation committed to upstream release/hotfix branches

[OpenNebulaSupport]

This concept of orphaned elements could also be extended to other objects in the OpenNebula setup.

• virtual networks (networks not declared in any VM template)
• VM templates (VM templates not referenced by oneflow service template)