Feat: Install Magnum client (#268)

The Magnum CLI client is missing.
diff --git a/roles/openstack_cli/defaults/main.yml b/roles/openstack_cli/defaults/main.yml
index 03d293c..8c94a52 100644
--- a/roles/openstack_cli/defaults/main.yml
+++ b/roles/openstack_cli/defaults/main.yml
@@ -16,5 +16,6 @@
 # List of packages to install for OpenStack CLI
 openstack_cli_packages:
   - python3-openstackclient
+  - python3-magnumclient
 
                                                                    # ]]]