RESOURCE-490 modification of the vpn connection both singular and plural resource

[soumyo13]

Description

1. Modify the docs for the Singular resource of vpn_connection resource.
2. Add the missing integration test for vpn_connections resources.
3. Add more tests in the integration test for vpn_connection resources.
4. Add more tests in the unit test for vpn_connection resources.
5. Modify the singular resource.
6. Adding the missing output in the output.tf file.

Issues Resolved

Need modifications in the singular resource.

Check List

Please fill box or appropriate ([x]) or mark N/A

• [x] New Terraform resources
• [x] Documentation provided or updated for resources
• [x] All Integration Tests pass
• [x] All Unit Tests pass
• [x] rake lint passes
• [x] All commits have been signed-off for the Developer Certificate of Origin. See https://github.com/chef/chef/blob/master/CONTRIBUTING.md#developer-certification-of-origin-dco

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