[cloudfront] OriginGroup doesn't register correctly as DefaultCacheBehavior target.

[LarsFronius]

When I create a Distribution with a defaultBehavior origin that points to a OriginGroup, the outcoming templates target ID points to the primaryOrigin of that OriginGroup instead of to the OriginGroup itself.

Reproduction Steps

   const originGroup = new OriginGroup({
      primaryOrigin: new S3Origin(someBucket, {originPath: "/"}),
      fallbackOrigin: new HttpOrigin(`${api.restApiId}.execute-api.${this.region}.amazonaws.com`, {originPath: `/${api.deploymentStage.stageName}/thumbnail`}),
      fallbackStatusCodes: [404]}
    )
    const cdn = new Distribution(this, 'cdn', {
      defaultBehavior: {
        origin: originGroup
      },
    })

This (and many other configurations) create a Distribution with the primary origins target ID registered as default target, rather than the origin group.

What did you expect to happen?

I expected that the originGroup would be targeted by default.

Environment

• **CLI Version : ** 1.57.0
• **Framework Version: ** 1.57.0
• Node.js Version: v14.7.0
• OS : macOS 10.15.5
• Language (Version): Typescript 3.7.2

Other

The issue was introduced as part of the original implementation of OriginGroups: #9360

I have a fix ready at master...LarsFronius:default-behaviour-origin-groups - let me know if this looks as expected and I'll create the PR.

/cc ing @skinny85 : Since you created the original implementation and have the most context, perhaps you are the best to give this a review. Thank you!

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This is 🐛 Bug Report

[skinny85]

@iliapolo I can take this one, since I was the one who implemented OriginGroups originally.

[iliapolo]

@skinny85 go for it 😃