Feat/config/generate (#136)

Implement config command & generate verb to write default configuration to file or stdout.

cli/cmd/configgenerate.go

@@ -0,0 +1,55 @@
+package cmd
+
+import (
+	"github.com/edgelesssys/constellation/cli/file"
+	"github.com/edgelesssys/constellation/internal/config"
+	"github.com/edgelesssys/constellation/internal/constants"
+	"github.com/spf13/afero"
+	"github.com/spf13/cobra"
+	"gopkg.in/yaml.v3"
+)
+
+func newConfigGenerateCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "generate",
+		Short: "Generate a default configuration file",
+		Long:  "Generate a default configuration file for your selected cloud provider.",
+		Args:  cobra.ExactArgs(0),
+		RunE:  runConfigGenerate,
+	}
+	cmd.Flags().StringP("file", "f", constants.ConfigFilename, "output file")
+
+	return cmd
+}
+
+type generateFlags struct {
+	file string
+}
+
+func runConfigGenerate(cmd *cobra.Command, args []string) error {
+	fileHandler := file.NewHandler(afero.NewOsFs())
+	return configGenerate(cmd, fileHandler)
+}
+
+func configGenerate(cmd *cobra.Command, fileHandler file.Handler) error {
+	flags, err := parseGenerateFlags(cmd)
+	if err != nil {
+		return err
+	}
+
+	if flags.file == "-" {
+		return yaml.NewEncoder(cmd.OutOrStdout()).Encode(config.Default())
+	}
+
+	return fileHandler.WriteYAML(flags.file, config.Default(), 0o644)
+}
+
+func parseGenerateFlags(cmd *cobra.Command) (generateFlags, error) {
+	file, err := cmd.Flags().GetString("file")
+	if err != nil {
+		return generateFlags{}, err
+	}
+	return generateFlags{
+		file: file,
+	}, nil
+}