Bump MCAPI to 0.18.0 (#1258)

diff --git a/images/magnum/Dockerfile b/images/magnum/Dockerfile
index 6abfca0..f5a9156 100644
--- a/images/magnum/Dockerfile
+++ b/images/magnum/Dockerfile
@@ -30,7 +30,7 @@
 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}