GatewayId is evil

Hey there,

I just spent a bit of time with AWS Support, so wanted to pass it on and get a type check in Route.

troposphere will gladly generate a template from this code:

``` python
nat = t.add_resource(ec2.NatGateway(
'NatGateway{}'.format(letter),
AllocationId=GetAtt(eip, 'AllocationId'),
SubnetId=az['subnet_id']
))
t.add_resource(ec2.Route(
'NatGateway{}Route'.format(letter),
GatewayId=Ref(nat),
DestinationCidrBlock='0.0.0.0/0',
RouteTableId=Ref(route_table_id),
))
```

and silly me didn't realize that `GatewayId` is only for IGWs and VGWs. For a NAT Gateway they have a separate field called `NatGatewayId` when creating a Route. The above should be:

``` python
t.add_resource(ec2.Route(
'NatGateway{}Route'.format(letter),
NatGatewayId=Ref(nat),
DestinationCidrBlock='0.0.0.0/0',
RouteTableId=Ref(route_table_id),
))
```

I think some troposhere resources do some type checking, right? Can we add a type check to `GatewayId` in `Route` to return an error message when someone references a `ec2.NatGateway`?

Start by locating the ec2.Route and ec2.NatGateway definitions and reviewing how existing resource property type checks work. Verify the distinction between GatewayId and NatGatewayId, then add coverage showing that a NatGateway reference in GatewayId produces an error while the NatGatewayId form remains valid.

Written by the indexing model from the issue text.