Add pod disruption budget for keystone

This is the initial pod disruption budget that will serve as the template
for all services in OpenStack-Helm.

Partially-Implements: blueprint add-pod-disruption-budgets

Change-Id: I67eeaa66257e793f77a089f3bc0dd4b700638c63

doc/source/operator/kubernetes.rst

@@ -7,3 +7,28 @@ Init-Containers
 
 Jobs
 ====
+
+Pod Disruption Budgets
+======================
+OpenStack-Helm leverages PodDistruptionBudgets to enforce quotas
+that ensure that a certain number of replicas of a pod are available
+at any given time.  This is particularly important in the case when a Kubernetes
+node needs to be drained.
+
+
+These quotas are configurable by modifying the ``minAvailable`` field
+within each PodDistruptionBudget manifest, which is conveniently mapped
+to a templated variable inside the ``values.yaml`` file.
+The ``min_available`` within each service's ``values.yaml`` file can be
+represented by either a whole number, such as ``1``, or a percentage,
+such as ``80%``.  For example, when deploying 5 replicas of a pod (such as
+keystone-api), using ``min_available: 3`` would enfore policy to ensure at
+least 3 replicas were running, whereas using ``min_available: 80%`` would ensure
+that 4 replicas of that pod are running.
+
+**Note:** The values defined in a PodDisruptionBudget may
+conflict with other values that have been provided if an operator chooses to
+leverage Rolling Updates for deployments.  In the case where an
+operator defines a ``maxUnavailable`` and ``maxSurge`` within an update strategy
+that is higher than a ``minAvailable`` within a pod disruption budget,
+a scenario may occur where pods fail to be evicted from a deployment.