AWS change default instance type for single-node control-plane

Background

• Starting with version 4.13 OCP is going to officially support Single
  Node clusters on AWS.

• The minimum documented OCP requirement for single-node control plane
  nodes is 8-cores and 16GiB of RAM

• The current default instance type chosen for AWS clusters by the
  installer is `xlarge` which is 4 cores and 16GiB of RAM

Issue

The default instance type the installer currently chooses for Single
Node Openshift clusters doesn't follow our documented minimum
requirements

Solution

When the number of replicas of the ControlPlane pool is 1, the installer
will now choose `2xlarge` instead of `xlarge`.

Caveat

`2xlarge` has 32GiB of RAM, which is twice as much as we need, but it's
the best we can do to meet the minimum single-node requirements, because
AWS doesn't offer a 16GiB RAM instance type with 8 cores.