cmd attribute of task is annotated as "not empty"

[jkrue]

The cmd attribute of an task is currently annotated as required. When running a docker container with default command it isn't necessary to set a cmd here.

[jkrue]

A adhoc bugfix is to remove the "not empty" annotation from the Task definition. This change has only a small effect on the external API. Better, but then with much more API incompatibility, is to go back to the previous Task definition:

• add an extra cmd attr for the container (which is optional)
• choice between cmd and container (one of both is mandatory)

I integrate the adHoc solution in beta.1.release

[pbelmann]

Sorry for the issue and thanks for the fix. Maybe we should schedule the fix that brings API incompatibility for beta.2.release.
@jkrue if you agree please update the label.

[jkrue]

@pbelmann You're right ... we should integrate the API incompatible fix in beta.2.release.