Bump MCAPI to 0.21.1 (#1609)

diff --git a/images/magnum/Dockerfile b/images/magnum/Dockerfile
index 41218f8..546b80f 100644
--- a/images/magnum/Dockerfile
+++ b/images/magnum/Dockerfile
@@ -19,7 +19,7 @@
 pip3 install \
     --constraint /upper-constraints.txt \
         /src/magnum \
-        magnum-cluster-api==0.20.0
+        magnum-cluster-api==0.21.1
 EOF
 
 FROM registry.atmosphere.dev/library/openstack-python-runtime:${RELEASE}