Conditionalize creation of Internet Gateway depending on whether public subnets are specified

[flostadler]

Internet Gateways are currently auto created even if no public subnets are configured.
For some customers with very strict security rules this causes compliance tests to fail.

This PR conditionalizes the creation of the Internet Gateway by checking if public subnets are specified. Only in this case the Internet Gateway is needed.

By conditionalizing the creation of the IGW the following breaking changes are introduced

• The internetGateway output of the VPC module changes to being optional. Users that use the output will need to add a null/undefined check when using it. This breaking change should only affect users with advanced VPC setups that manually use the output of the IGW
• The Internet Gateway will be deleted for users that are already using an older version of the VPC component resource when they have no public subnets configured. In case they've configured public subnets outside of the stack and are using the implicitly created IGW, this will not have destructive effects because Internet Gateways cannot be detached from VPCs if they're in use (if there's at least one EIP in the VPC).

This fixes #947