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>

pkg/rbd/nodeserver.go

@@ -62,10 +62,11 @@ func (ns *NodeServer) NodeStageVolume(ctx context.Context, req *csi.NodeStageVol
 
 	volID := req.GetVolumeId()
 
-	cr, err := util.GetUserCredentials(req.GetSecrets())
+	cr, err := util.NewUserCredentials(req.GetSecrets())
 	if err != nil {
 		return nil, status.Error(codes.Internal, err.Error())
 	}
+	defer cr.DeleteCredentials()
 
 	isLegacyVolume := false
 	volName, err := getVolumeName(req.GetVolumeId())