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/cephfs/kubernetes/csi-cephfsplugin.yaml

@@ -90,6 +90,8 @@ spec:
               mountPath: /dev
             - name: ceph-csi-config
               mountPath: /etc/ceph-csi-config/
+            - name: keys-tmp-dir
+              mountPath: /tmp/csi/keys
       volumes:
         - name: mount-cache-dir
           emptyDir: {}
@@ -121,3 +123,7 @@ spec:
         - name: ceph-csi-config
           configMap:
             name: ceph-csi-config
+        - name: keys-tmp-dir
+          emptyDir: {
+            medium: "Memory"
+          }