feat(proc-cmdline): Parse /proc/cmdline for cloud-config-url

If the --from-proc-cmdline flag is given to coreos-cloudinit, the local
/proc/cmdline file will be parsed for a cloud-config-url

datasource/datasource.go

@@ -1,6 +1,31 @@
 package datasource
 
+import (
+	"io/ioutil"
+	"net/http"
+)
+
 type Datasource interface {
 	Fetch() ([]byte, error)
 	Type()  string
 }
+
+func fetchURL(url string) ([]byte, error) {
+	client := http.Client{}
+	resp, err := client.Get(url)
+	if err != nil {
+		return []byte{}, err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode / 100 != 2 {
+		return []byte{}, nil
+	}
+
+	respBytes, err := ioutil.ReadAll(resp.Body)
+	if err != nil {
+		return nil, err
+	}
+
+	return respBytes, nil
+}