Update json module to jsonutils

oslo project provide jsonutils, and octavia use it in many place[1],
this PS to update the remained json moudule to oslo jsonutils for
consistency.

[1]: https://github.com/openstack/octavia-tempest-plugin/search?utf8=%E2%9C%93&q=jsonutils&type=

Change-Id: I1cf966595bbabd3e33b1e33033dfb6a8b16381f8
diff --git a/requirements.txt b/requirements.txt
index 815be94..c9f839c 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -9,6 +9,7 @@
 octavia-lib>=1.0.0 # Apache-2.0
 oslo.config>=5.2.0 # Apache-2.0
 oslo.log>=3.36.0  # Apache-2.0
+oslo.serialization!=2.19.1,>=2.18.0 # Apache-2.0
 oslo.utils>=3.33.0  # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0
 python-barbicanclient>=4.5.2 # Apache-2.0