Extend image override logic to Daemonsets #280

Describe the bug
The forthcoming knative-serving release will deploy the activator as a daemonset, but our image override logic only replaces images in Deployments currently.

Expected behavior
Images for Daemonsets should be subject to the same override logic as for Deployments

To Reproduce
Try overriding the activator images and see that its Daemonset image changes.

Knative release version
Post 0.12

Additional context
This is the change upstream: knative/serving#6624

This issue might lend some justification to #277 as it would be handy to be able to test a released operator with an unreleased manifest.