sam pipelines with CodePipeline don't let you set trusted account

I wish SAM pipelines let you have a proper account structure :
* Tools account hosting the pipeline
* Stages accounts hosting the payload
As recommended in AWS best practices

As of today the proposed pipeline assume pipeline will be deploy in the same account as stage 1 which might not be the best structure to propose to customer.

This is especially conditionned by the trust relationship set in pipeline execution role.

### Proposal

I see 2 options:
* Add a new set of question around what is going to be the "Tools" account
* Add a simple question for each stage regarding what will be the account to trust