F-322: add opennebula_templates datasource

[treywelsh]

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Description

Add templates datasource which allows to manage a filtered list of templates.

References

Close #322

New or Affected Resource(s)

• data opennebula_templates

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]

name_regex attribute added, and a code duplication reduced