Deployment strategies

We could offer deployment strategies (best practices?) to the user. So e.g. instead of doing an all or nothing promote, we could offer a few options like:

* test deploy - don't promote
* canary deploy - promote new version and redirect 1% traffic
* full deploy - promote new version and redirect all traffic

Test and full could be a default strategy, while users could define others (like canary) and they could be shown in the menu, where we have a single "Deploy to App Engine" item, we could have a submenu with the defined strategies.