Support a schema in plan for vars

[emilingerslev]

It is hard and error-prone to use the vars system. What vars can be used? Did I misspell some of them?
Add some level of schema for the vars, so required vars and optional vars are easily validated

[Crevil]

I think the simplest approach would be to use JSON schema specifications for the plans and parse the plan yaml files against it.

The https://github.com/go-openapi/validate project support validating an input string against a schema. A simple example is shown in the Godoc documentation of https://godoc.org/github.com/go-openapi/validate#AgainstSchema. The documentation states that it can validate both json and yaml formats.

// Example using encoding/json as unmarshaller
var schemaJSON = `
{
"properties": {
    "name": {
        "type": "string",
        "pattern": "^[A-Za-z]+$",
        "minLength": 1
    }
	},
"patternProperties": {
	  "address-[0-9]+": {
     "type": "string",
     "pattern": "^[\\s|a-z]+$"
	  }
},
"required": [
    "name"
],
	"additionalProperties": false
}`

schema := new(spec.Schema)
json.Unmarshal([]byte(schemaJSON), schema)

input := map[string]interface{}{}

// JSON data to validate
inputJSON := `{"name": "Ivan","address-1": "sesame street"}`
json.Unmarshal([]byte(inputJSON), &input)

// strfmt.Default is the registry of recognized formats
err := validate.AgainstSchema(schema, input, strfmt.Default)
if err != nil {
    fmt.Printf("JSON does not validate against schema: %v", err)
} else {
    fmt.Printf("OK")
}