Allow reading k0s config from a separate or multidoc YAML document

[kke]

Fixes #812

• Multiple --config statements, globs such as config/**/*.yaml or directories as in --config configs/ are now allowed.
• spec.k0s.config can now be defined in a separate config in the standard k0s configuration format with apiVersion: k0s.k0sproject.io/v1beta1 and kind: ClusterConfig.
• The separate k0s config can also be contained in the same multidoc YAML configuration file as the k0sctl.yaml when separated with the --- document separator.

Multidoc configuration example:

apiVersion: k0sctl.k0sproject.io/v1beta1
kind: cluster
spec:
  hosts:
    - role: single
      ssh:
        address: 10.0.0.1
  k0s:
    version: "v1.31.3+k0s.0"
---
apiVersion: k0s.k0sproject.io/v1beta1
kind: ClusterConfig
metadata:
  name: k0s
spec:
  api:
    address: 192.168.68.104
    k0sApiPort: 9443
    port: 6443