Comment 1 for bug 1176661

You have this:
    "floating_ip_DNSserver": {
      "Type": "OS::Quantum::FloatingIP",
      "Properties": {
        "floating_network_id": { "Ref" : "ExtNetUuid" }
      }
    },

and your error is this:
| stack_status_reason | Resource FloatingIPAssociation |
| | "floating_ip_assoc_DNSserver" failed with: |
| | QuantumClientException: External network 524ac1ab-f8d3 |
| | -47cd-b0df-cfdc7072b9cf is not reachable from subnet |
| | 71145fd6-d5ce-4d10-8e91-46792a8b6e3a. Therefore, |
| | cannot associate Port 7c

It looks like ExtNetUuid is not reachable from the subnet you created, or you actually meant to do this in floating_ip_DNSserver:
        "floating_network_id": { "Ref" : "network" }