Feature: PreCondition / TemplateCondition

Nobody has claimed this yet.

**Summary** We need a feature `PreCondition` or `TemplateCondition` that allows for specifying a condition on a resource being included in template building.

**Use Case**: We currently use `Condition: NotInChina` for resources that are not applicable to China.

This breaks for resources that are not at all available in China, e.g. KMS.
The template is validated before any conditions are applied, which fails if the resource does not exist in AWS-CN. We need a condition that removes these resources before sending it to CFN.

**Example**:

```
KmsAccess: {
Action: ‘Kms:Decrypt’,
Effect: ‘Allow’,
Resource: ‘kms:keys:*’,
PreCondition: ‘NotInChina’
}
```

The resource should be removed from the template object, *before* being submitted to Cloudformation.

This may require a change in /[cfn-config](https://github.com/mapbox/cfn-config) also, since cloudfriend itself is region/deploy-agnostic I think.

cc @mapbox/platform

Start by tracing cloudfriend's template-building and validation flow, then inspect the linked /cfn-config boundary to determine which project owns region-specific behavior. Done means resources using the proposed condition are removed from the template object before submission to CloudFormation, including the KMS example.

Written by the indexing model from the issue text.