Upgrade Kubernetes version deployed by RKE to v1.19.6
Manual system_images assignment in rke's cluster config
is dropped as for any particular kubernetes release Rancher
project holds a predefined, static map connecting Rancher's
Kubernetes version with particular system images, so it's
enough to define k8s version.
Ref.: https://rancher.com/docs/rke/latest/en/config-options/system-images/
Change-Id: I027589c86500bbc5260caaa4aef7065d23d2241c
Issue-ID: OOM-2686
Signed-off-by: Bartek Grzybowski <b.grzybowski@partner.samsung.com>