nixos/tests/ec2: return to passing state

Updates required:

- Use vpc image format (new default, supported by Amazon)
- Pass full image filename to makeEc2Test
- Increase memory allocation for nixos-rebuild
- Set a networking.hostName for services.httpd
- Add appropriate escaping in literal userdata

While I'm here, try to make it fail fast.

nixos/tests/openstack-image.nix

@@ -17,7 +17,7 @@ let
         ../modules/testing/test-instrumentation.nix
         ../modules/profiles/qemu-guest.nix
       ];
-    }).config.system.build.openstackImage;
+    }).config.system.build.openstackImage + "/nixos.qcow2";
 
   sshKeys = import ./ssh-keys.nix pkgs;
   snakeOilPrivateKey = sshKeys.snakeOilPrivateKey.text;