Add reno for unified limits

This first version should be considered as a preview, that will become
more useable once more of the transition tooling has been implemented.

Given the massive change, it seems prudent to get as much of this into
operators hands as quickly as possible, so we can get some early
feedback.

blueprint unified-limits-nova

Change-Id: I80ccca500d1a2eb8f19b5843a0d0d337c583e104

Author: JohnGarbutt

releasenotes/notes/bp-unified-limits-656b55863df22e16.yaml

@@ -0,0 +1,43 @@
+---
+other:
+  - |
+    This release includes work in progress support for Keystone's unified
+    limits. This should not be used in production. It is included so we can
+    collect early feedback from operators around the performance of the new
+    limits system. There is currently no way to export your existing quotas
+    and import them into Keystone. There is also no proxy API to allow you
+    to update unified limits via Nova APIs. All the update APIs behave as if
+    you are using the noop driver when the unified limits quota driver is
+    configured.
+
+    When you enable unified limits, those are configured in Keystone against
+    the Nova endpoint, using the names:
+
+    * ``class:VCPU``
+    * ``servers``
+    * ``class:MEMORY_MB``
+    * ``server_metadata_items``
+    * ``server_injected_files``
+    * ``server_injected_file_content_bytes``
+    * ``server_injected_file_path_bytes``
+    * ``server_key_pairs``
+    * ``server_groups``
+    * ``server_group_members``
+
+    All other resources classes requested via flavors are also now supported as
+    unified limits. Note that nova configuration is ignored, as the default
+    limits come from the limits registered for the Nova endpoint in Keystone.
+
+    All previous quotas other than ``cores``, ``instances`` and ``ram`` are
+    still enforced, but the limit can only be changed globally in Keystone as
+    registered limits. There are no per project or per user overrides
+    possible.
+
+    Work in progress support for Keystone's unified limits
+    can be enabled via ``[quota]/driver=nova.quota.UnifiedLimitsDriver``
+
+    A config option ``[workarounds]unified_limits_count_pcpu_as_vcpu`` is
+    available for operators who require the legacy quota usage behavior where
+    VCPU = VCPU + PCPU. Note that if ``PCPU`` is specified in the flavor
+    explicitly, it will be expected to have its own unified limit registered
+    and PCPU usage will *not* be merged into VCPU usage.