Allow assigning kubernetes cluster to project

[MauriceArikoglu]

Currently all resources that support being added to projects allow specifying a project_id argument which then creates the resource associated to a project.

The digitalocean kubernetes cluster resource does not support this argument. We need to create a second project_resources resource that adds the kubernetes cluster to the project manually which is prone to issues. It would be great if associating the cluster with a project by passing the project_id as argument was possible.

[bruno-leitao]

This is extremely important, as this argument is *required in a manual configuration.