Update os_magnum role pin

The magnum source repository unfortunately is not
adhering to the global requirements management process
in that it has requirements specified in the repo which
are not in the global requirements repo.

As a result of the requirement being missing, those
requirements are not constrained by upper-constraints.
That, in turn, causes the repo build to fail to build
the magnum venv due to incompatible requirements.

This patch brings in the updated role SHA which pins
kubernetes and related dependencies and the release note
to communicate the change.

Change-Id: I49312a568ff6ce74497f00eec288166707e33fc0

ansible-role-requirements.yml

@@ -93,7 +93,7 @@
 - name: os_magnum
   scm: git
   src: https://git.openstack.org/openstack/openstack-ansible-os_magnum
-  version: 46a857c938f3d5c83a91bdd73f67e7a206b301d1
+  version: fd806ccddec31e36234ce979906448501db47083
 - name: os_trove
   scm: git
   src: https://git.openstack.org/openstack/openstack-ansible-os_trove

releasenotes/notes/kubernetes-pin-56768037d47ebc73.yaml

@@ -0,0 +1,13 @@
+---
+issues:
+  - |
+    For all ocata releases prior to OpenStack-Ansible 15.1.16,
+    several of the magnum requirements were not constrained.
+    This resulted in the kubernetes version changing over time
+    unexpectedly, rather than by intention. Most recently this
+    lack of constraint causes the repo build to fail when trying
+    to build the magnum venv due to incompatible requirements.
+
+    The kubernetes version deployed with ocata is now constrained
+    to version 2.0.0, matching the same version as deployed in
+    all pike releases.