User Story:

As an openshift-instlal user I want to be able to specify AWS subnets with roles. 

Acceptance Criteria:

Description of criteria:

• Installconfig has installconfig.platform.aws.vpc.subnets field
• vpc.subnets conforms to API defined in the enhancement
• godoc/oc explain text is written and generated (see explain docs)

(optional) Out of Scope:

Validations will be handled in a different card.

Engineering Details:

• API for installconfig field is defined in https://github.com/openshift/enhancements/blob/30f44ee0cd57dc4ba3b72e10c0b8f1614970d0e0/enhancements/installer/aws-lb-subnet-selection.md

• • the type for subnets is:
    • id - string
    • roles - slice of SubnetRoles
• the list of subnet roles is defined in the enhancement. they include ClusterNode, EdgeNode, ControlPlaneExternalLBSubnetRole, etc...
•  

This requires/does not require a design proposal.
This requires/does not require a feature gate.