Adds skaffold based setup

[plkokanov]

/kind enhancement

What this PR does / why we need it:
Adds skaffold based setup for local development and removes the no longer necessary rsyslog-relp-up-kind.sh script.

Additionally, the image field for the extension's ControllerRegistration is now flattened in the form of image: repository:tag so that it can be modified by skaffold with the following config:

resourceSelector:
  allow:
  # instruct skaffold to inject the built image reference into the image field in our ControllerDeployment
  - groupKind: ControllerDeployment.core.gardener.cloud
    image: [".*"]

This appropriate change is also done in the charts for the extension so that the ControllerRegistration is generated properly.

Finally, some no longer necessary values have been removed from the charts.

Which issue(s) this PR fixes:
Part of #2

Special notes for your reviewer:

Release note:

Added `skaffold` based setup for local development. The extension can now be deployed in a local gardener kind cluster with the `make extension-up` or `make extension-dev` commands. The extension can be deleted by executing `make extension-down`.

The image field in the `ControllerRegistration` of the extension and in the `gardener-extension-shoot-rsyslog-relp` charts has been flattened so that it is in the form `image: repository:tag`.