Make deployment idempotent per service version

[carnesen]

Problem

Currently we push a new version for every service for every deployment. This has a few disadvantages:

• deployment takes a very long time since we are generally doing 8x more work than we need to be every time
• It makes it hard to know which version of an app is deployed because all we can see is the auto-generated version
• Because we are deploying so many versions we run into limits and have to go around cleaning up

Solution

• Use gcloud app deploy's --version flag to set the version explicitly when creating a new version
• Only deploy the current version if it does not already exist