Refactor manifests generation

- introduce manifestgen pkg, to be consumed by the CLI and Terraform provider
- consolidate defaults in manifestgen/install pkg
- introduce Manifest as the returning type of manifest generation
- add helper function to Manifest for writing multi-doc YAMLs on disk

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

pkg/manifestgen/manifest.go

@@ -0,0 +1,49 @@
+/*
+Copyright 2020 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package manifestgen
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path"
+	"path/filepath"
+)
+
+// Manifest holds the data of a multi-doc YAML
+type Manifest struct {
+	// Relative path to the YAML file
+	Path string
+	// Content in YAML format
+	Content string
+}
+
+// WriteFile writes the YAML content to a file inside the the root path.
+// If the file does not exist, WriteFile creates it with permissions perm,
+// otherwise WriteFile overwrites the file, without changing permissions.
+func (m *Manifest) WriteFile(rootDir string) (string, error) {
+	if err := os.MkdirAll(path.Join(rootDir, filepath.Dir(m.Path)), os.ModePerm); err != nil {
+		return "", fmt.Errorf("unable to create dir, error: %w", err)
+	}
+
+	filePath := path.Join(rootDir, m.Path)
+	if err := ioutil.WriteFile(filePath, []byte(m.Content), os.ModePerm); err != nil {
+		return "", fmt.Errorf("unable to write file, error: %w", err)
+	}
+
+	return filePath, nil
+}