Machinepool should use OCI_MANAGED_KUBERNETES_VERSION ENV (#375)

Using KUBERNETES_VERSION instead of OCI_MANAGED_KUBERNETES_VERSION
will have a version mismatch and the managed nodes will fail
to join.

Author: joekr

test/e2e/data/infrastructure-oci/v1beta2/cluster-template-managed/machine-pool.yaml

@@ -16,7 +16,7 @@ spec:
         apiVersion: infrastructure.cluster.x-k8s.io/v1beta2
         kind: OCIManagedMachinePool
         name: ${CLUSTER_NAME}-mp-0
-      version: ${KUBERNETES_VERSION}
+      version: ${OCI_MANAGED_KUBERNETES_VERSION}
 ---
 apiVersion: infrastructure.cluster.x-k8s.io/v1beta2
 kind: OCIManagedMachinePool
@@ -52,7 +52,7 @@ spec:
         apiVersion: infrastructure.cluster.x-k8s.io/v1beta2
         kind: OCIManagedMachinePool
         name: ${CLUSTER_NAME}-mp-1
-      version: ${KUBERNETES_VERSION}
+      version: ${OCI_MANAGED_KUBERNETES_VERSION}
 ---
 apiVersion: infrastructure.cluster.x-k8s.io/v1beta2
 kind: OCIManagedMachinePool