cluster/kube: always enable flexdriver

Documentation says [1] this is disabled by default in 1.1, but that
documentation kinda lies [2].

[1] - https://github.com/rook/rook/blob/235d5a384baa68ee688b2eb870e0e3b5b14ad0e3/Documentation/flexvolume.md#ceph-flexvolume-configuration

[2] - https://github.com/rook/rook/commit/64e28af74195befb449af81fe234143db5b0d2b4#diff-d1eb5cba50e3770b61ccd3c730cd40514053e1da0233dfe09b5e7967e76a2a6cL424-L425

Change-Id: Ia92c99e137ed751db62c0f56d42c4901986d0bb8
diff --git a/cluster/kube/lib/rook.libsonnet b/cluster/kube/lib/rook.libsonnet
index 4acf80d..a3cd3d3 100644
--- a/cluster/kube/lib/rook.libsonnet
+++ b/cluster/kube/lib/rook.libsonnet
@@ -585,7 +585,8 @@
                                     NODE_NAME: kube.FieldRef("spec.nodeName"),
                                     POD_NAME: kube.FieldRef("metadata.name"),
                                     POD_NAMESPACE: kube.FieldRef("metadata.namespace"),
-                                    ROOK_CSI_KUBELET_DIR_PATH: "/var/lib/kubernetes"
+                                    ROOK_CSI_KUBELET_DIR_PATH: "/var/lib/kubernetes",
+                                    ROOK_ENABLE_FLEX_DRIVER: "true",
                                 },
                             },
                         },