flannel: added flannel support and helper to make dropins

fleet, flannel, and etcd all generate dropins from config.
To reduce code duplication, factor out a helper to do that.

config/config.go

@@ -30,11 +30,12 @@ import (
 type CloudConfig struct {
 	SSHAuthorizedKeys []string `yaml:"ssh_authorized_keys"`
 	Coreos            struct {
-		Etcd   Etcd   `yaml:"etcd"`
-		Fleet  Fleet  `yaml:"fleet"`
-		OEM    OEM    `yaml:"oem"`
-		Update Update `yaml:"update"`
-		Units  []Unit `yaml:"units"`
+		Etcd    Etcd    `yaml:"etcd"`
+		Flannel Flannel `yaml:"flannel"`
+		Fleet   Fleet   `yaml:"fleet"`
+		OEM     OEM     `yaml:"oem"`
+		Update  Update  `yaml:"update"`
+		Units   []Unit  `yaml:"units"`
 	} `yaml:"coreos"`
 	WriteFiles        []File   `yaml:"write_files"`
 	Hostname          string   `yaml:"hostname"`

config/flannel.go

@@ -0,0 +1,9 @@
+package config
+
+type Flannel struct {
+	EtcdEndpoint string `yaml:"etcd-endpoint" env:"FLANNELD_ETCD_ENDPOINT"`
+	EtcdPrefix   string `yaml:"etcd-prefix"   env:"FLANNELD_ETCD_PREFIX"`
+	IPMasq       string `yaml:"ip-masq"       env:"FLANNELD_IP_MASQ"`
+	SubnetFile   string `yaml:"subnet-file"   env:"FLANNELD_SUBNET_FILE"`
+	Iface        string `yaml:"interface"     env:"FLANNELD_IFACE"`
+}