feat(config-drive): Add support for reading user-data from config-drive

The -config-drive flag tells cloudinit to read the user-data from
within the config-drive (./openstack/latest/user-data).

datasource/configdrive.go

@@ -0,0 +1,27 @@
+package datasource
+
+import (
+	"io/ioutil"
+	"os"
+	"path"
+)
+
+type configDrive struct {
+	path string
+}
+
+func NewConfigDrive(path string) *configDrive {
+	return &configDrive{path}
+}
+
+func (self *configDrive) Fetch() ([]byte, error) {
+	data, err := ioutil.ReadFile(path.Join(self.path, "openstack", "latest", "user_data"))
+	if os.IsNotExist(err) {
+		err = nil
+	}
+	return data, err
+}
+
+func (self *configDrive) Type() string {
+	return "cloud-drive"
+}