Fix handling of block devices with NoDevice=true

Spot instances have been seen to fail to launch if this attribute is set to
true.

We can just skip the volume from the instance block device mappings if this
field is set, instead of actually propagating the true value that
eventually has the same result.

core/instance.go

@@ -486,13 +486,12 @@ func (i *instance) convertBlockDeviceMappings(lc *launchConfiguration) []*ec2.Bl
 			}
 		}
 
-		// it turns out that the noDevice field needs to be converted from bool to
-		// *string
-		if lcBDM.NoDevice != nil {
-			ec2BDM.NoDevice = aws.String(fmt.Sprintf("%t", *lcBDM.NoDevice))
+		// handle the noDevice field directly by skipping the device if set to true
+		if lcBDM.NoDevice != nil && *lcBDM.NoDevice {
+			continue
 		}
-
 		bds = append(bds, ec2BDM)
+
 	}
 	return bds
 }

core/instance_test.go

@@ -1367,36 +1367,28 @@ func Test_instance_convertBlockDeviceMappings(t *testing.T) {
 			want: []*ec2.BlockDeviceMapping{},
 		},
 		{
-			name: "instance-store only",
+			name: "instance-store only, skipping one of the volumes from the BDMs",
 			lc: &launchConfiguration{
 				LaunchConfiguration: &autoscaling.LaunchConfiguration{
 					BlockDeviceMappings: []*autoscaling.BlockDeviceMapping{
 						{
 							DeviceName:  aws.String("/dev/ephemeral0"),
 							Ebs:         nil,
-							NoDevice:    aws.Bool(false),
+							NoDevice:    aws.Bool(true),
 							VirtualName: aws.String("foo"),
 						},
 						{
 							DeviceName:  aws.String("/dev/ephemeral1"),
 							Ebs:         nil,
-							NoDevice:    aws.Bool(false),
 							VirtualName: aws.String("bar"),
 						},
 					},
 				},
 			},
 			want: []*ec2.BlockDeviceMapping{
-				{
-					DeviceName:  aws.String("/dev/ephemeral0"),
-					Ebs:         nil,
-					NoDevice:    aws.String("false"),
-					VirtualName: aws.String("foo"),
-				},
 				{
 					DeviceName:  aws.String("/dev/ephemeral1"),
 					Ebs:         nil,
-					NoDevice:    aws.String("false"),
 					VirtualName: aws.String("bar"),
 				},
 			},
@@ -1410,7 +1402,6 @@ func Test_instance_convertBlockDeviceMappings(t *testing.T) {
 						{
 							DeviceName:  aws.String("/dev/ephemeral0"),
 							Ebs:         nil,
-							NoDevice:    aws.Bool(false),
 							VirtualName: aws.String("foo"),
 						},
 						{
@@ -1436,7 +1427,6 @@ func Test_instance_convertBlockDeviceMappings(t *testing.T) {
 				{
 					DeviceName:  aws.String("/dev/ephemeral0"),
 					Ebs:         nil,
-					NoDevice:    aws.String("false"),
 					VirtualName: aws.String("foo"),
 				},
 				{