Added remote fleet support for anthos modules

[CPL-markus]

It should be possible to enable the ACM feature on a GKE cluster that has its fleet in a separate project.

Therefore an additonal variable hub_project_id was introduced to reference the fleet project id. If it is empty (default), the project_id variable will be used instead.

[apeabody]

Thanks for the contribution @CPL-markus!

Can you please make your change in the source template as described here: https://github.com/terraform-google-modules/terraform-google-kubernetes-engine/blob/master/CONTRIBUTING.md#templating

Checking submodule's files generation
diff -r '--exclude=.terraform' '--exclude=.kitchen' '--exclude=.git' /workspace/modules/acm/README.md /tmp/tmp.AIQao951VV/workspace/modules/acm/README.md
91a92
> | hub\_project\_id | The project in which the GKE Hub belongs. Defaults to GKE cluster project\_id. | `string` | `""` | no |
Error: submodule's files generation has not been run, please run the
'make build' command and commit changes
/usr/local/bin/task_helper_functions.sh: line 30: DELETE_AT_EXIT: readonly variable
Checking for documentation generation
diff -r '--exclude=.terraform' '--exclude=.kitchen' '--exclude=autogen' '--exclude=*.tfvars' '--exclude=*metadata.yaml' /workspace/modules/acm/README.md /tmp/tmp.mBX4FUQN9b/generate_docs/workspace/modules/acm/README.md
91a92
> | hub\_project\_id | The project in which the GKE Hub belongs. Defaults to GKE cluster project\_id. | `string` | `""` | no |
Error: Documentation generation has not been run, please run the
'make docker_generate_docs' command and commit the above changes.