pin Openstackclient version

Change-Id: I61608fa73a27f284c570759432c12b0447c06ab5
diff --git a/tcp_tests/requirements.txt b/tcp_tests/requirements.txt
index 759a449..e9c2917 100644
--- a/tcp_tests/requirements.txt
+++ b/tcp_tests/requirements.txt
@@ -27,5 +27,5 @@
 # For Queens: https://github.com/openstack/requirements/blob/stable/queens/global-requirements.txt
 python-heatclient>=1.10.0
 python-glanceclient>=2.8.0
-python-openstackclient>=3.12.0
+python-openstackclient==3.18.0
 keystoneauth1>=3.3.0