amazonec2: correct mis-handling of 'none' VPC

Without this change,

docker-machine create \
    --driver amazonec2 \
    --amazonec2-security-group parity-security-group \
    --amazonec2-instance-type t2.medium \
    --amazonec2-access-key ... \
    --amazonec2-secret-key ... \
    --amazonec2-root-size 128 \
    parity-node

yields the program output

Error with pre-create check: "unable to find a subnet in the zone: us-e
ast-1a"

which comes from drivers/amazonec2/amazonec2.go:506.

But that message is misleading.  After sprinkling in some
`fmt.Println`s, I discovered that the missing piece of info is not
actually the subnet, but rather the VPC ID, which is "none".

With this change, that same command now yields:

Error setting machine configuration from flags provided: amazonec2 driv
er requires either the --amazonec2-subnet-id or --amazonec2-vpc-id opti
on or an AWS Account with a default vpc-id

Signed-off-by: F. Eugene Aumson <feuGeneA@github.com>

Author: feuGeneA

drivers/amazonec2/amazonec2.go

@@ -1235,7 +1235,11 @@ func (d *Driver) getDefaultVPCId() (string, error) {
 
 	for _, attribute := range output.AccountAttributes {
 		if *attribute.AttributeName == "default-vpc" {
-			return *attribute.AttributeValues[0].AttributeValue, nil
+			value := *attribute.AttributeValues[0].AttributeValue
+			if value == "none" {
+				return "", errors.New("default-vpc is 'none'")
+			}
+			return value, nil
 		}
 	}
 

drivers/amazonec2/amazonec2_test.go

@@ -227,6 +227,31 @@ func TestDefaultVPCIsMissing(t *testing.T) {
 	assert.Empty(t, vpc)
 }
 
+func TestDefaultVPCIsNone(t *testing.T) {
+	driver := NewDriver("machineFoo", "path")
+	attributeName := "default-vpc"
+	vpcName := "none"
+	driver.clientFactory = func() Ec2Client {
+		return &fakeEC2WithDescribe{
+			output: &ec2.DescribeAccountAttributesOutput{
+				AccountAttributes: []*ec2.AccountAttribute{
+					{
+						AttributeName: &attributeName,
+						AttributeValues: []*ec2.AccountAttributeValue{
+							{AttributeValue: &vpcName},
+						},
+					},
+				},
+			},
+		}
+	}
+
+	vpc, err := driver.getDefaultVPCId()
+
+	assert.EqualError(t, err, "default-vpc is 'none'")
+	assert.Empty(t, vpc)
+}
+
 func TestGetRegionZoneForDefaultEndpoint(t *testing.T) {
 	driver := NewCustomTestDriver(&fakeEC2WithLogin{})
 	driver.awsCredentialsFactory = NewValidAwsCredentials