Support running operator in a disconnected environment

[tnozicka]

We should support running operator in a disconnected environment. The main issue there is the inability to access public container image registries. The operator uses a few embedded images to spin up additional stacks, like ScyllaDBMonitoring. At the same time we need to be able to reference exactly the same image but a different registry. Unfortunately we can't use shas as their computation is registry dependent. At the same time letting users to freely adjust the image would bump into incompatibilities and bugs which we also don't want. One way this can be done is a "release image" that contains the sha references and a mirror command that copies the images to the disconnected registry and updates the "release image" - e.g. as done in https://docs.openshift.com/container-platform/4.16/installing/disconnected_install/installing-mirroring-disconnected.html

### TODO:
- [ ] Disconnected install R&D
- [ ] Add e2e periodics with disconnected environment
- [ ] Add docs on how to run in a disconnected environment