fix(etcd): fix runtime panic when etcd section is missing.

The etcd code tries to assign ee["name"] even when the map was never
defined and assigning to an uninitialized map causes a panic.

initialize/etcd.go

@@ -31,6 +31,10 @@ func (ee EtcdEnvironment) String() (out string) {
 // Unit creates a Unit file drop-in for etcd, using any configured
 // options and adding a default MachineID if unset.
 func (ee EtcdEnvironment) Unit(root string) (*system.Unit, error) {
+	if ee == nil {
+		return nil, nil
+	}
+
 	if _, ok := ee["name"]; !ok {
 		if machineID := system.MachineID(root); machineID != "" {
 			ee["name"] = machineID

initialize/etcd_test.go

@@ -152,3 +152,15 @@ Environment="ETCD_NAME=node007"
 		t.Fatalf("File has incorrect contents")
 	}
 }
+
+func TestEtcdEnvironmentWhenNil(t *testing.T) {
+	// EtcdEnvironment will be a nil map if it wasn't in the yaml
+	var ee EtcdEnvironment
+	if ee != nil {
+		t.Fatalf("EtcdEnvironment is not nil")
+	}
+	u, err := ee.Unit("")
+	if u != nil || err != nil {
+		t.Fatalf("Unit returned a non-nil value for nil input")
+	}
+}