F-485: add datasource opennebula vms

[treywelsh]

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Description

Introduce opennebula_virtual_machines datasource, and add some fixes around opennebula_templates.

References

Close #485

New or Affected Resource(s)

• data_opennebula_virtual_machines

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

[frousselet]

Wait while merging 1.4.1 PR first.

[github-actions]

This pull request is stale because it has been open for 30 days with no activity and it is not in a milestone. Remove 'status: stale' label or comment, or this will be closed in 5 days.