Allow passing initial manifest to apply

[weibeld]

Add an additional string input variable which may contain either a YAML or JSON Kubernetes manifest, or a URL to a manifest. This manifest is then applied from the master node (kubectl apply) immediately after bootstrapping the cluster.

This would allow, for example, to automatically install a CNI plugin in the cluster.

Notes:

• It's important to also check the exit code of this command (see Detect failure in bootstrap scripts #21) and fail the Terraform run if it is unsuccessful (to detect that e.g. an invalid manifest has been passed).
• kubectl apply -f is supposed to accept either a YAML or JSON file or a URL. So the value of the input variable can be passed as-is to kubectl apply.