This commit adds support to discover garage instances running in
kubernetes.
Once enabled by setting `kubernetes_namespace` and
`kubernetes_service_name` garage will create a Custom Resources
`garagenodes.deuxfleurs.fr` with nodes public key as the resource name.
and IP and Port information as spec in the namespace configured by
`kubernetes_namespace`.
For discovering nodes the resources are filtered with the optionally set
`kubernetes_service_name` which sets a label
`garage.deuxfleurs.fr/service` on the resources.
This allows to separate multiple garage deployments in a single
namespace.
the `kubernetes_skip_crd` variable allows to disable the creation of the
CRD by garage itself. The user must deploy this manually.
