[stable/zed] Bump MCAPI to 0.18.0 (#1272)

This is an automated cherry-pick of #1258
/assign mnaser
diff --git a/images/magnum/Dockerfile b/images/magnum/Dockerfile
index 103adfe..09948bf 100644
--- a/images/magnum/Dockerfile
+++ b/images/magnum/Dockerfile
@@ -32,7 +32,7 @@
 pip3 install \
     --constraint /upper-constraints.txt \
         /src/magnum \
-        magnum-cluster-api==0.16.0
+        magnum-cluster-api==0.18.0
 EOF
 
 FROM registry.atmosphere.dev/library/openstack-python-runtime:${RELEASE}