feat: allow windows custom images per agent pool

ci: don't run vmss-prototype tests for release (Azure#4458)

chore: rev 18.04-LTS VHD to 2021.05.24 (Azure#4438)

fix: vmas with ephemeral os should use aligned AS sku (Azure#4436)

fix: ephemeral VMAS should work (Azure#4420)

Update 1.21.0 components

fix: install runc rc92

test: add containerd no outbound test scenario (Azure#4371)

feat: don't install nvidia drivers if nvidia-device-plugin is disabled (

Azure#4358)

Add 1.18.17/1.19.9/1.20.5 (Azure#4347)