Allowing adding a label to Keda's namespace

[aqeelat]

To solve this issue: https://keda.sh/troubleshooting/istio-keda-faileddiscoverycheck/

I think this can be done by allowing users to pass in --set keda.disableIstioInjection=true.
I can open a PR for this change.

[aqeelat]

@benjaminhuo you're feedback is much appreciated here.

[benjaminhuo]

@benjaminhuo you're feedback is much appreciated here.

@wrongerror what do you think?