refactor(ec2): API cleanups, more control over UserData (aws#2954)

Clean up EC2 APIs, hide all free-floating subclasses and introduce static factory functions (`Peer` and `Port`).

Start extensibility campaign for UserData. There are now classes for Linux and Windows-formatted UserData, and users can supply their own instance and/or subclass when creating an ASG. Simplify the user of `IMachineImage`.

Fixes aws#2674.

BREAKING CHANGES:

* **ec2**: `TcpPort` etc have moved to factory method
  on the `Port` class.
* **ec2**: `InstanceTypePair` has moved to factory method
  on `InstanceType`.
* **ec2**: `natDependencies` and `internetDependencies` removed,
  replaced with `internetConnectivityEstablished`.
* **ec2**: removed `selectSubnetIds`.
* **ec2**: removed `isPublicSubnets`.
* **ec2**: `VpnConnection` is now a `Resource`.
* **ec2**: `VpcNetworkProvider` is no longer available directly,
  use `Vpc.fromLookup()` instead.
* **ec2**: `routeTableId` is now a `IRouteTable` object.
* **ec2**: `AnyIPv4` etc have move as factory functions onto `Peer`.
* **ec2**: `IMachineImageSource` => `IMachineImage`.
* **ec2** : `MachineImage` => `MachineImageConfig`.

Author: rix0rrr

NOTICE

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 AWS Cloud Development Kit (AWS CDK)
 Copyright 2018-2018 Amazon.com, Inc. or its affiliates. All Rights Reserved.
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