UI App model validation blocks scale and suspend operations

We are using Marathon's v2 API to create a new app, we are using executor and container.image params to specify docker executor and image. Since, we are using executor and image, we are not providing any 'cmd' string. And, Marathon is able to create the app successfully.

But, from Marathon's UI we are not able to perform scale or suspend operations, because the model validation logic in UI expects 'cmd' to be a non-empty string. 

This pull request, modifies the validation logic to do an additional check for non-empty executor field and container.image fields, when cmd is empty, before erroring out.