Make `ps` command work before first successful deploy

[ipmb]

When first setting up an app, it's not uncommon to have it fail for various reasons (not enough resources, missing config vars, failed healthchecks, etc). When it's in that state, apppack ps doesn't work because the necessary metadata to determine the services hasn't been written to DynamoDB yet. At a minimum, we need to show a better error message to the user than could not find DDB item APP#... DEPLOYSTATUS#..., but ideally, we make this command work in that scenario.

Two possible options:

1. Let the CLI figure out what services are running by scanning the cluster for name/tags that match.
2. Have the backend write that metadata earlier in the process (after build, before deploy)