Changing args to cmd triggers validation error

[bobrik]

I'm running master.

1. Create app with container and args set.
2. Decide that you want to pass $PORT, therefore you need to use cmd.
3. Remove args, add cmd. Deploy.

AppDefinition must either contain one of 'cmd' or 'args', and/or a 'container'.

It is possible to remove app and deploy with cmd and container.

cc @drexin, possibly happened in #1201.

[drexin]

I think before #1201 it worked by accident. We have to provide a way to unset fields. PUT should override the whole app, which it currently doesn't and the behaviour it currently has needs to be exposed by a new PATCH endpoint.

[aquamatthias]

The problem is, that you can not express unsetting values the way we provide the API.
See #1418