Use the AWS instance type in the cfn_init scenario

All the other resources in the template are of the AWS kind and this
will help with merging I9d01c5e7de630df5058429353fa17b3536ee8807.

This was derived from tempest change
I61346b569ed16d8a3c7610a85351ef723287fd83

Change-Id: Icd15a9863c00e63409af635b24938bec3f125e16
Co-Authored-By: Tomas Sedovic <tomas@sedovic.cz>
diff --git a/scenario/test_server_cfn_init.py b/scenario/test_server_cfn_init.py
index a98747c..cc5d9d2 100644
--- a/scenario/test_server_cfn_init.py
+++ b/scenario/test_server_cfn_init.py
@@ -37,7 +37,7 @@
             'flavor': self.conf.instance_type,
             'image': self.conf.image_ref,
             'timeout': self.conf.build_timeout,
-            'network': net['id'],
+            'subnet': net['subnets'][0],
         }
 
         # create the stack