Variables in policy doc

Rather than having separate policy docs for each stage, just have one with variables. Like so:

**./.chalice/policy.json**

```json
{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Action": [
"s3:DeleteObject",
"s3:GetObject",
"s3:ListBucket",
"s3:PutObject"
],
"Resource": [
"arn:aws:s3:::my-bucket-${stage}"
]
}
]
}
```

^^^ See the `${stage}`. That would be neat.

Start with the proposed ./.chalice/policy.json example and review how the current separate policy documents are handled for each stage. Done means a single policy document can use ${stage} to produce the appropriate stage-specific resource values, with the behavior verified by the project’s existing checks.

Written by the indexing model from the issue text.