Bump Magnum Cluster API to 0.28.0

Change-Id: Ib76a20b23cba4e3b0e7a55495728ee14c5da6c70
diff --git a/images/magnum/Dockerfile b/images/magnum/Dockerfile
index 94e7190..01d15bc 100644
--- a/images/magnum/Dockerfile
+++ b/images/magnum/Dockerfile
@@ -21,7 +21,7 @@
 pip3 install \
     --constraint /upper-constraints.txt \
         /src/magnum \
-        magnum-cluster-api==0.27.0
+        magnum-cluster-api==0.28.0
 EOF
 
 FROM openstack-python-runtime