fix(aws-ec2): change maxAZs default for VPCs to 3

For most users, default deployments of a VPC in `us-east-1` would fail
because that region has more AZs (6) than the allowed EIP allocations in
an acccount (5).

This is a very poor experience. Plus, it is wasteful since there is
not much additional value in using more than 3 AZs for a VPC.

Fixes #996.

packages/@aws-cdk/aws-ec2/lib/vpc.ts

@@ -58,7 +58,7 @@ export interface VpcNetworkProps {
    * If the region has more AZs than you want to use (for example, because of EIP limits),
    * pick a lower number here. The AZs will be sorted and picked from the start of the list.
    *
-   * @default All AZs in the region
+   * @default 3
    */
   maxAZs?: number;
 
@@ -315,9 +315,9 @@ export class VpcNetwork extends VpcNetworkBase implements cdk.ITaggable {
 
     this.availabilityZones = new cdk.AvailabilityZoneProvider(this).availabilityZones;
     this.availabilityZones.sort();
-    if (props.maxAZs != null) {
-       this.availabilityZones = this.availabilityZones.slice(0, props.maxAZs);
-    }
+
+    const maxAZs = props.maxAZs !== undefined ? props.maxAZs : 3;
+    this.availabilityZones = this.availabilityZones.slice(0, maxAZs);
 
     this.vpcId = this.resource.vpcId;
     this.dependencyElements.push(this.resource);

packages/@aws-cdk/aws-ec2/test/test.vpc.ts

@@ -233,6 +233,25 @@ export = {
       }));
       test.done();
     },
+
+    "maxAZs defaults to 3 if unset"(test: Test) {
+      const stack = getTestStack();
+      new VpcNetwork(stack, 'VPC');
+      expect(stack).to(countResources("AWS::EC2::Subnet", 6));
+      expect(stack).to(countResources("AWS::EC2::Route", 6));
+      for (let i = 0; i < 6; i++) {
+        expect(stack).to(haveResource("AWS::EC2::Subnet", {
+          CidrBlock: `10.0.${i * 32}.0/19`
+        }));
+      }
+      expect(stack).to(haveResourceLike("AWS::EC2::Route", {
+        DestinationCidrBlock: '0.0.0.0/0',
+        NatGatewayId: { },
+      }));
+
+      test.done();
+    },
+
     "with maxAZs set to 2"(test: Test) {
       const stack = getTestStack();
       new VpcNetwork(stack, 'VPC', { maxAZs: 2 });