Release manifest updates in relation to corner case use-cases (#60)

Changes:

- Rancher version convention was changed from `v2.10.0` to `2.10.0` to better map to the actual version in the upstream helm chart repo which is without the `v` prefix.

- Rancher's `postDelete` hook has been disabled - done to ensure that we will not hit a corner case where:

   1. The Rancher helm chart upgrade fails, because of a core component not yet being ready
   2. The `helm-controller` schedules a `helm uninstall` which deletes the Rancher Helm release and triggers the `postDelete` hook.
   3. The problematic core component is up and running, so `helm-controller` schedules a `helm install` with the new version.
   4. Due to insufficient resources, or network connection (or other unforeseen problems), the `postDelete` hook is still running and it wrongly removes the new Rancher installation resulting in a missing rancher from the cluster after an upgrade.

The `postDelete` hook ensures that no accidental delete of the Rancher application will happen during an upgrade over a machine with fewer resources.

Reviewed-on: #60
Reviewed-by: Denislav Prodanov <dprodanov@noreply.src.opensuse.org>
Reviewed-by: Nicolas Belouin <nbelouin@noreply.src.opensuse.org>
Reviewed-by: Atanas Dinov <atanasdinov@noreply.src.opensuse.org>
Co-authored-by: Ivo Petrov <ivo.petrov@suse.com>
Co-committed-by: Ivo Petrov <ivo.petrov@suse.com>

release-manifest-image/release_manifest.yaml

@@ -88,8 +88,11 @@ spec:
         - prettyName: Rancher
           releaseName: rancher
           chart: rancher
-          version: v2.10.0
+          version: 2.10.0
           repository: https://releases.rancher.com/server-charts/stable
+          values:
+            postDelete:
+              enabled: false
         - prettyName: Longhorn
           releaseName: longhorn
           chart: longhorn