Generate image pull secret at bootstrap

Add an optional flag called `--registry-creds` to the bootstrap
command for generating an image pull secret for container images
stored in private registries.

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

cmd/flux/bootstrap_github.go

@@ -191,6 +191,7 @@ func bootstrapGitHubCmdRun(cmd *cobra.Command, args []string) error {
 		Namespace:              *kubeconfigArgs.Namespace,
 		Components:             bootstrapComponents(),
 		Registry:               bootstrapArgs.registry,
+		RegistryCredential:     bootstrapArgs.registryCredential,
 		ImagePullSecret:        bootstrapArgs.imagePullSecret,
 		WatchAllNamespaces:     bootstrapArgs.watchAllNamespaces,
 		NetworkPolicy:          bootstrapArgs.networkPolicy,