Azure: Update installer to use NAT gateway for egress

User Story:

As a administrator, I want to be able to:

•  Allow NAT Gateway as outboundType for clusters in Azure

so that I can achieve

• Outbound access without exhausting SNAT ports

Acceptance Criteria:

Description of criteria:

• NAT gateway as an outboundType in install-config

Engineering Details:

• https://issues.redhat.com/browse/RFE-3386
• https://github.com/openshift/installer/pull/6933

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