AWLS2-432: switch defaults for image URL and NAT gateway options.

[wilderj]

Summary

This PR makes two changes to the default deployment options:

• NAT gateways are once again the default. After much internal discussion and testing, we've decided that most customers are better off with a simpler deployment experience at the expense of additional cost (~$35 per month per region).
• We are switching to an Azure-native ACR repository for pulling our container image. This was a long planned update that also helps avoid throttling issues when using NAT gateways.

How did you test this change?

We already have our internal canary environment running with these changes. To be sure, I will create another new temporary deployment to validate the updated defaults.