Split image registry and repository from Helm chart

[QuantumEnigmaa]

Currently, the image in sloth's deployment is set to {{ .Values.admissionController.image.repository }}:{{ .Values.admissionController.image.tag }} in the templates.

However, in some cases, one may need to change the registry name without changing the repository itself. This would be especially useful in cases where one has the same repository in multiple registries (GCR, GHCR, Harbor...) and has to use a specific registry for a chart deployed in a specific environment.

This PR's goal is thus to give the possibility to change the image's registry without having to modify the repository.