Use --keyfile option to pass keys to all Ceph CLIs

Every Ceph CLI that is invoked at present passes the key via the
--key option, and hence is exposed to key being displayed on
the host using a ps command or such means.

This commit addresses this issue by stashing the key in a tmp
file, which is again created on a tmpfs (or empty dir backed by
memory). Further using such tmp files as arguments to the --keyfile
option for every CLI that is invoked.

This prevents the key from being visible as part of the argument list
of the invoked program on the system.

Fixes: #318

Signed-off-by: ShyamsundarR <srangana@redhat.com>

deploy/rbd/kubernetes/csi-rbdplugin.yaml

@@ -90,6 +90,8 @@ spec:
             - name: mountpoint-dir
               mountPath: /var/lib/kubelet/pods
               mountPropagation: "Bidirectional"
+            - name: keys-tmp-dir
+              mountPath: /tmp/csi/keys
       volumes:
         - name: socket-dir
           hostPath:
@@ -122,3 +124,7 @@ spec:
         - name: ceph-csi-config
           configMap:
             name: ceph-csi-config
+        - name: keys-tmp-dir
+          emptyDir: {
+            medium: "Memory"
+          }