AWS API raises an exception if both AZ and VPCZoneIdentifier params are empty. mock that exception, fix

tests to follow that pattern.

tests/test_autoscaling/utils.py

@@ -0,0 +1,17 @@
+import boto3
+from moto import mock_ec2
+
+
+@mock_ec2
+def setup_networking():
+    ec2 = boto3.resource('ec2', region_name='us-east-1')
+    vpc = ec2.create_vpc(CidrBlock='10.11.0.0/16')
+    subnet1 = ec2.create_subnet(
+        VpcId=vpc.id,
+        CidrBlock='10.11.1.0/24',
+        AvailabilityZone='us-east-1a')
+    subnet2 = ec2.create_subnet(
+        VpcId=vpc.id,
+        CidrBlock='10.11.2.0/24',
+        AvailabilityZone='us-east-1b')
+    return {'vpc': vpc.id, 'subnet1': subnet1.id, 'subnet2': subnet2.id}