trustor user should be updated on stack update

If a user account is removed, Heat trusts where that user is the trustor will no longer work. This means that e.g. autoscaling applications will break when the user who originally deployed them leaves the organisation.

To allow users to resolve this, we should generate a new trust when a different user updates the stack. This means that e.g. a colleague can take over maintenance of the application, and future autoscaling will happen in their name instead of the original owner's.