allow to use Java engine if helm installation defaults to executor

[RafalSkolasinski]

If seldon-core is installed using helm with --set executor.enabled=true one cannot deploy a single POD using engine.

In opposite situation, when above is set to false, one can enforce executor with annotations:

spec:
  annotations:
    seldon.io/executor: "true"

P.S. Not sure how much it is important as we are going to deprecate Java engine.

[seldondev]

Issues go stale after 30d of inactivity.
Mark the issue as fresh with /remove-lifecycle stale.
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/lifecycle stale

[axsaucedo]

/remove-lifecycle stale
/priority p1
Adding high priority given that this will be quite key for backwards compatibility support