Adding regex based exports replacement

[ammokhov]

Issue #, if available:

Description of changes:

Adding a regex based exports replacement, as it is a less expensive manipulation. The change finds {{ variable }} pattern that jinja supports but verifies that variable is compliant with stack export naming convention.

Example:

overrides.json

{
    "CREATE": {
        "/SubnetId": "{{ subnetID }}"
    }
}

rendered overrides.json

{
    "CREATE": {
        "/SubnetId": "subnet-0bc6136e"
    }
}

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.