Fix IstioD causing the Cortex cluster to no longer be reachable

[RobertLucian]

Fixes #2332

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checklist:

• run make test and make lint
• test manually (i.e. build/push all images, restart operator, and re-deploy APIs)

[miguelvr]

AFAIK there are a more places referring to prometheus in the default namespace

From the top of my head, look for the autoscaler, activator, cluster health commands. But it is better to do a wide search for anything using the word "default"