B #484: Add image field to disk schema

[jaimecb]

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Description

This PR allows defining disks by image name in VM templates

References

Closes #484

New or Affected Resource(s)

• opennebula_template
• opennebula_virtual_machine

Checklist

• I have created an issue and I have mentioned it in References
• My code follows the style guidelines of this project (use go fmt)
• My changes generate no new warnings or errors
• I have updated the unit tests and they pass succesfuly
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation (if needed)
• I have updated the changelog file