ValueError: Resource does not exist: api_gateway_custom_domain

Hello,

I've got a fairly simple app/API going here but when trying to use the custom domain portion of Chalice I get the error noted in the subject of this post. My configuration is fairly simple, and I've tried literally copying the documented example on both the Chalice docs and the AWS doc examples and simply replacing the domain and the certificate ARN and have had absolutely no joy.

```
{
"app_name": "xxxxx",
"environment_variables": {
"ddb_table": "xxxx"
},
"api_gateway_stage": "api",
"autogen_policy": true,
"stages": {
"dev": {
"tags": {
"Stage":"DEV"
}
},
"prod": {
"api_gateway_custom_domain": {
"domain_name": "xxxx",
"certificate_arn": "xxxx"
},
"tags": {
"Stage":"PRD"
}
}
}
}
```
Any ideas what might be causing this? The domain and cert are configured correctly (and I've tested manually configuring them in the AWS console and all works fine). This is running on my local machine in a virtual env, deployments and deployed folder exist and working as normal.

Start by reproducing a deployment with the shown prod-stage custom-domain configuration, then trace Chalice's custom-domain deployment entry point and the source of the ValueError. Compare the failing configuration with the documented example and the manually working API Gateway setup; done means the deployment accepts the configuration and creates the custom domain without the missing-resource error.

Written by the indexing model from the issue text.