Apply additional kube manifests from configs to cluster

[kke]

Additional feature for #812

Any encountered configuration files that are in the "kubernetes resource" format (contains apiVersion and kind fields) will be applied to the cluster using kubectl apply as the last step before disconnect during a k0sctl apply action.

Example YAML:

apiVersion: k0sctl.k0sproject.io/v1beta1
kind: cluster
spec:
  hosts:
    - role: single
      ssh:
        address: 10.0.0.1
---
apiVersion: v1
kind: Pod
metadata:
  name: hello
spec:
  containers:
  - name: hello
    image: nginx:alpine
    ports:
    - containerPort: 80

[jnummelin]

One minor question only, otherwise LGTM

[jnummelin]

Aren't we loading all otherConfigs from a single yaml doc? In this case, what does otherConfig.Filename() refer to?

[kke]

k0sctl accepts multiple --configs or a dir --config configs/ (which gets converted to configs/**/*.{yml,yaml} or a glob pattern --config "configs/k0s*.yaml", not only a single multidoc yaml.

The manifest reader tries to keep track of their origins for displaying messages, but if it came from stdin or it doesn't know the filename for some other reason, it either just calls it "manifest" (during preliminary reading/unmarshaling) or calls it fmt.Sprintf("%s-%s-%d.yaml", safeFn(rd.APIVersion), safeFn(rd.Kind), time.Now().UnixNano()) (once the header has been parsed).