The value of `image_registry` is not being used in the cloud monitoring agent

[ocofaigh]

Deploying the agent in a cluster which has no public gateway access is failing with ImagePullBackOff errors like:

Back-off pulling image "quay.io/sysdig/.....

quay.io should not be getting used. It should be using the value passed for image_registry which defaults to icr.io/ext/sysdig/agent

We should update the tests to ensure there is no public gateways on the cluster nodes so that we catch such an issue

[jor2]

#64