Fix terraform0.13 errors (#7077)

* [terraform/aws] Fix Terraform >=0.13 warnings

Terraform >=0.13 gives the following warning:

```
Warning: Interpolation-only expressions are deprecated
```

The fix was tested as follows:
```
rm -rf .terraform && terraform0.12.26 init && terraform0.12.26 validate
rm -rf .terraform && terraform0.13.5 init && terraform0.13.5 validate
rm -rf .terraform && terraform0.14.3 init && terraform0.14.3 validate
```
which gave no errors nor warnings.

* [terraform/openstack] Fixes for Terraform >=0.13

Terraform >=0.13 gives the following error:
```
Error: Failed to install providers
Could not find required providers, but found possible alternatives:
  hashicorp/openstack -> terraform-provider-openstack/openstack
```

This patch fixes these errors.

This fix was tested as follows:
```
rm -rf .terraform && terraform0.12.26 init && terraform0.12.26 validate
rm -rf .terraform && terraform0.13.5 init && terraform0.13.5 validate
rm -rf .terraform && terraform0.14.3 init && terraform0.14.3 validate
```
which gave no errors nor warnings for Terraform 0.13.5 and Terraform
0.14.3. Unfortunately, 0.12.x gives a harmless warning, but
with 0.14.3 out the door, I guess we need to move on.

* [terraform/packet] Fixes for Terraform >=0.13

This fix was tested as follows:
```
export PACKET_AUTH_TOKEN=blah-blah
rm -rf .terraform && terraform0.12.26 init && terraform0.12.26 validate
rm -rf .terraform && terraform0.13.5 init && terraform0.13.5 validate
rm -rf .terraform && terraform0.14.3 init && terraform0.14.3 validate
```

Errors are gone, but warnings still remain. It is impossible to please
all three versions of Terraform.

* Add tests for Terraform >=0.13