test/provision: update k8s test versions to 1.14.9 and 1.15.6
test/provision: update k8s test versions to 1.14.9 and 1.15.6

Signed-off-by: André Martins <bc9800b9d52a24cce72a73dd528afed53f10e5fc@cilium.io>

cilium-team/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,tklauser/cilium,cilium-team/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,tgraf/cilium