Add ability to re-use existing API Gateway with deploy

Currently, the only way to re-use an existing API Gateway endpoint in a `chalice deploy` command is to have the deployed resource index stored in the `.chalice/deployed/.json` file. In a multiple user environment, this works fine as long as the `.json` file is committed to the GIT repo.

However, when trying to use `chalice deploy` in a CI/CD pipeline (such as GitLab CI), the first release must commit the new `.json` file to the repository. Otherwise, each deployment creates a new API Gateway endpoint. Other resources, such as Lambda function or IAM role, which have a consistent name per account and stage, are correctly re-deployed.

**Proposal**:

Add an additional option, `--reuse-api-gateway` to the `chalice deploy` command. Usage would look something like:

```
chalice deploy --reuse-api-gateway --stage prod
```

**Expected behavior without option**:

Chalice would deploy exactly as it currently does

**Expected behavior with option**:

Chalice would interrogate the API Gateway to find the API ID that matches the name of the api being deployed and re-use it, rather than creating a new one.

Start by tracing the `chalice deploy` command and how `.chalice/deployed/.json` is used to identify existing resources. Review the deployment path for API Gateway and the GitLab CI scenario described in the issue. Done means the new `--reuse-api-gateway` option reuses an API Gateway matching the API name, while deployment without it behaves unchanged.

Written by the indexing model from the issue text.