add resolvable configmap/secret refs

[cbarbian-sap]

Add the following types:

type ConfigMapReference struct {
	// +required
	Name string            `json:"name"`
}

type ConfigMapKeyReference struct {
	// +required
	Name string `json:"name"`
	// +optional
	Key   string `json:"key,omitempty"`
}

type SecretReference struct {
	// +required
	Name string            `json:"name"`
}

type SecretKeyReference struct {
	// +required
	Name string `json:"name"`
	// +optional
	Key   string `json:"key,omitempty"`
}

that may be used in component spec definitions. References configmaps/secrets (resp. keys) will be auto-loaded in the post-read hook phase of each reconcile pass; loaded content can be retrieved by the Data() resp. Value() methods of the ref types.

Notes:

• component specs must use pointers to the above reference types
• absence of referenced configmaps/secrets (resp. keys) will lead to a reconcile error (leading to reconcile backoff)
• fallback keys can be specified as struct tag on the according field, such as

  type KubeConfigSpec struct {
    SecretRef SecretKeyReference `json:"secretRef" fallbackKeys:"value,value.yaml,value.yml"`
  }

[TheBigElmo]

lgtm