title: CloudFormation - Security Group VPC issue
body: <p>I have a template which creates an ELB and attaches an existing subnet within a VPC. This creates just fine but when I then update my stack and add a security group with a VpcId property with a value equal to the existing VPC ID in which my attached subnet belongs the stack fails with the following error: </p>
  
  <blockquote>
    <p>"You have specified two resources that belong to different networks"</p>
  </blockquote>
  
  <p>If I remove the VpcId property from my security group it creates it in my default VPC and the stack creation works. I cannot understand why this can be because the security group has a relationship to the ELB in the specified ingress rules - </p>
  
  <pre><code>"IpProtocol": "tcp",
              "FromPort": "8000",
              "ToPort": "8010",
              "SourceSecurityGroupOwnerId": {
                "Fn::GetAtt": [
                  "ElasticLoadBalancer",
                  "SourceSecurityGroup.OwnerAlias"
                ]
              },
  </code></pre>
  
  <p>I cannot explicitly state the VPC ID on the ELB as it has no such property, only Subnet or AZ.</p>