Document node and label specific configurations

This PS adds documentation for node and label specific configurations.

Change-Id: I2bb02bfa028a61b2d8a9206eaff305590664946f

doc/source/devref/index.rst

@@ -13,3 +13,4 @@ Contents:
    pod-disruption-budgets
    upgrades
    fluent-logging
+   node-and-label-specific-configurations

doc/source/devref/node-and-label-specific-configurations.rst

@@ -0,0 +1,106 @@
+Node and label specific configurations
+--------------------------------------
+
+There are situations where we need to define configuration differently for
+different nodes in the environment. For example, we may require that some nodes
+have a different vcpu_pin_set or other hardware specific deltas in nova.conf.
+
+To do this, we can specify overrides in the values fed to the chart. Ex:
+
+.. code-block:: yaml
+
+    conf:
+      nova:
+        DEFAULT:
+          vcpu_pin_set: "0-31"
+          cpu_allocation_ratio: 3.0
+      overrides:
+        nova_compute:
+          labels:
+          - label:
+              key: compute-type
+              values:
+              - "dpdk"
+              - "sriov"
+            conf:
+              nova:
+                DEFAULT:
+                  vcpu_pin_set: "0-15"
+          - label:
+              key: another-label
+              values:
+              - "another-value"
+            conf:
+              nova:
+                DEFAULT:
+                  vcpu_pin_set: "16-31"
+          hosts:
+          - name: host1.fqdn
+            conf:
+              nova:
+                DEFAULT:
+                  vcpu_pin_set: "8-15"
+          - name: host2.fqdn
+            conf:
+              nova:
+                DEFAULT:
+                  vcpu_pin_set: "16-23"
+
+Note that only one set of overrides is applied per node, such that:
+
+1. Host overrides supercede label overrides
+2. The farther down the list the label appears, the greater precedence it has.
+   e.g., "another-label" overrides will apply to a node containing both labels.
+
+Also note that other non-overridden values are inherited by hosts and labels with overrides.
+The following shows a set of example hosts and the values fed into the configmap for each:
+
+1. ``host1.fqdn`` with labels ``compute-type: dpdk, sriov`` and ``another-label: another-value``:
+
+   .. code-block:: yaml
+
+    conf:
+      nova:
+        DEFAULT:
+          vcpu_pin_set: "8-15"
+          cpu_allocation_ratio: 3.0
+
+2. ``host2.fqdn`` with labels ``compute-type: dpdk, sriov`` and ``another-label: another-value``:
+
+   .. code-block:: yaml
+
+    conf:
+      nova:
+        DEFAULT:
+          vcpu_pin_set: "16-23"
+          cpu_allocation_ratio: 3.0
+
+3. ``host3.fqdn`` with labels ``compute-type: dpdk, sriov`` and ``another-label: another-value``:
+
+   .. code-block:: yaml
+
+    conf:
+      nova:
+        DEFAULT:
+          vcpu_pin_set: "16-31"
+          cpu_allocation_ratio: 3.0
+
+4. ``host4.fqdn`` with labels ``compute-type: dpdk, sriov``:
+
+   .. code-block:: yaml
+
+    conf:
+      nova:
+        DEFAULT:
+          vcpu_pin_set: "0-15"
+          cpu_allocation_ratio: 3.0
+
+5. ``host5.fqdn`` with no labels:
+
+   .. code-block:: yaml
+
+    conf:
+      nova:
+        DEFAULT:
+          vcpu_pin_set: "0-31"
+          cpu_allocation_ratio: 3.0