Enhancement: support the stack to work with private VPCs

Although the documentation states that Internet connectivity is _required_ for the deployment of the stack, it can be changed so the stack could work without this requirement. The majority of software (with the exception of AWS CLI v2) is hosted on S3, so a private VPC with the S3 VPC endpoint works just fine (if one comments out the forceful update of AWS CLI v2). I've raised the corresponding feature request in AWS CLI v2 repo to re-consider the release hosting location to be moved to S3 too.

Given that the auxiliary EC2 instance is spawned from Amazon Linux 2 which already has AWS CLI installed, do we really need to forcibly re-install the package?