F-359: manage template vectors

[treywelsh]

Community Note

• Please vote on this issue by adding a 👍 reaction to the original issue to help the community and maintainers prioritize this request
• Please do not leave "+1" or other comments that do not add relevant new information or questions, they generate extra noise for PR followers and do not help prioritize the request

Description

Until now, it's possible to pass custom (key=value) pairs to an OpenNebula resource via the provider.
However it doesn't allow to pass a custom vector (keyvec = [key=value, ... ] ).
The goal of this issue is to allow to pass custom vectors to resources.

Limitation: for simplicity it will be only possible to manage a vector with a unique name

References

Close #359

New or Affected Resource(s)

• opennebula_group
  TODO: all other resources

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

[treywelsh]

I had to fix an SDK bug in the provider due to: hashicorp/terraform-plugin-sdk#588
The acceptance test were failing due to an empty template_section returned by the SDK. I filtered out this empty item.

[frousselet]

As seen, tags should be named elements.