[aws][local-zone][installer] create edge machine pool on installer

USER STORY:

from enhancement

• As a cluster admin, I want the ability to specify a set of subnets on the AWS
  Local Zone locations to deploy worker nodes, so I can create custom applications
  to deliver low latency to my end users.

• As a cluster admin, I would like to create a cluster extending worker nodes to the
  edge of the AWS cloud provider with Local Zones, so I can create custom applications
  to deliver low latency to my end users.

• As a cluster admin, I would like to control the workloads running on the edge
  compute pool, and keep it dedicated for applications which is latency sensitive,
  so I can have cost-effective infrastrucure, avoiding mixing workloads in different locations.

• As a cluster admin, I would like to select existing subnets from the local and
  the parent region zones, to install a cluster, so I can manage my custom networks with
  my automation.

DESCRIPTION:

Required:

The installer, when installing a cluster in existing VPC with Local Zone subnets, must:

• support "edge" compute pool on the install-config.yaml 
• the AWS Local Zone subnets must be classified as "Edge subnets"
• The installer must create one Machine Set by "Edege subnets"

Nice to have:

...

ACCEPTANCE CRITERIA:

• PR approved by the installer team
• PR approved by SPLAT team

ENGINEERING DETAILS:

Enhancement Proposal 1232: https://github.com/openshift/enhancements/pull/1232/files#