re-define noEngine annotation

[chengcheng-pei]

this is original codes:
_, noEngine := p.Annotations[machinelearningv1.ANNOTATION_NO_ENGINE]

if machinelearningv1.ANNOTATION_NO_ENGINE in annotations, noEngine is true, otherwise, it is false.

This is confusing. we need check the value of p.Annotations[machinelearningv1.ANNOTATION_NO_ENGINE] instead of whether it exists in the annotations map or not.

there is same concern about _, hasSeparateEnginePod := mlDep.Spec.Annotations[machinelearningv1.ANNOTATION_SEPARATE_ENGINE].

Please leave comments, so that I can update the pr: #1970

[ukclivecox]

I see you have created an initial PR. I will update to WIP and link to this issue. I agree its presently usinging exitence rather than boolean which makes more sense I think.