Add kubernetes as required provider

[abuechler]

We get some warnings when using the cast.ai provider:

│ There is no explicit declaration for local provider name "kubernetes" in module.dev-castai, so Terraform is assuming you mean to pass a configuration for "hashicorp/kubernetes".
│
│ If you also control the child module, add a required_providers entry named "kubernetes" with the source address "hashicorp/kubernetes".