metadata: simplify merging of metadata

Add an internal field for CloudConfig to make it easier to distinguish.
Instead of creating two CloudConfigs and merging them, just merge the
metadata into the existing CloudConfig.

config/config.go

@@ -27,14 +27,13 @@ import (
 // directly to YAML. Fields that cannot be set in the cloud-config (fields
 // used for internal use) have the YAML tag '-' so that they aren't marshalled.
 type CloudConfig struct {
-	SSHAuthorizedKeys []string `yaml:"ssh_authorized_keys"`
-	CoreOS            CoreOS   `yaml:"coreos"`
-	WriteFiles        []File   `yaml:"write_files"`
-	Hostname          string   `yaml:"hostname"`
-	Users             []User   `yaml:"users"`
-	ManageEtcHosts    EtcHosts `yaml:"manage_etc_hosts"`
-	NetworkConfigPath string   `yaml:"-"`
-	NetworkConfig     string   `yaml:"-"`
+	SSHAuthorizedKeys []string  `yaml:"ssh_authorized_keys"`
+	CoreOS            CoreOS    `yaml:"coreos"`
+	WriteFiles        []File    `yaml:"write_files"`
+	Hostname          string    `yaml:"hostname"`
+	Users             []User    `yaml:"users"`
+	ManageEtcHosts    EtcHosts  `yaml:"manage_etc_hosts"`
+	Internal          Internals `yaml:"-"`
 }
 
 type CoreOS struct {
@@ -47,6 +46,10 @@ type CoreOS struct {
 	Units     []Unit    `yaml:"units"`
 }
 
+type Internals struct {
+	NetworkConfig []byte
+}
+
 func IsCloudConfig(userdata string) bool {
 	header := strings.SplitN(userdata, "\n", 2)[0]