Add missing property on magnum plugin

Since Magnum 1.15 users can set 'floating_ip_enabled' property on
Magnum cluster templates [1].

This patch ensures the possibility to enable the mentioned property
within Magnum tasks.

[1]: https://github.com/openstack/magnum/blob/master/magnum/objects/cluster_template.py#L41

Change-Id: Icd77d369dfc6a6f3f4837772fe3724c2bac1e0b3

CHANGELOG.rst

@@ -26,6 +26,7 @@ Added
 * Added barbican scenarios.
 * Added octavia scenarios.
 * Support for osprofiler config in Devstack plugin.
+* Added property 'floating_ip_enabled' in magnum cluster_templates context.
 
 Changed
 ~~~~~~~

rally_openstack/contexts/magnum/cluster_templates.py

@@ -92,6 +92,9 @@ class ClusterTemplateGenerator(context.Context):
             },
             "master_lb_enabled": {
                 "type": "boolean"
+            },
+            "floating_ip_enabled": {
+                "type": "boolean"
             }
         },
         "required": ["image_id", "external_network_id", "coe"],