Convert driver supported capabilities to compute node provider traits

This builds on the ProviderTree work in the compute driver and
resource tracker to take the supported capabilities from a driver and
turn those into standard traits on the compute node resource provider.

This is a simple way to expose in a REST API (Placement in this case)
what a compute node, via its driver, supports.

This is also something easy that we can do in lieu of a full-blown
compute capabilities REST API in nova, which we've talked about for
years but never actually done anything about.

We can later build on this to add a request filter which will mark
certain types of boot-from-volume requests as requiring specific
capabilities, like for volume multiattach and tagged devices.

Any traits provided by the driver will be automatically added during
startup or a periodic update of a compute node:

    https://pasteboard.co/I3iqqNm.jpg

Similarly any traits later retracted by the driver will be
automatically removed.

However any traits associated with capabilities which are
inappropriately added to or removed from the resource provider by the
admin via the Placement API will not be reverted until the compute
service's provider cache is reset.

The new call graph is shown in this sequence diagram:

    https://pasteboard.co/I25qICd.png

Co-Authored-By: Adam Spiers <aspiers@suse.com>

Related to blueprint placement-req-filter
Related to blueprint expose-host-capabilities

Change-Id: I15364d37fb7426f4eec00ca4eaf99bec50e964b6

1 files changed, 26 insertions, 0 deletions

diff --git a/releasenotes/notes/driver-capabilities-to-traits-152eb851cd016f4d.yaml b/releasenotes/notes/driver-capabilities-to-traits-152eb851cd016f4d.yaml
new file mode 100644
index 0000000000..da60ed4a12
--- /dev/null
+++ b/releasenotes/notes/driver-capabilities-to-traits-152eb851cd016f4d.yaml
@@ -0,0 +1,26 @@
+---
+features:
+  - |
+    Compute drivers now expose capabilities via traits in the
+    Placement API.  Capabilities must map to standard traits defined
+    in `the os-traits project
+    <https://docs.openstack.org/os-traits/latest/>`_; for now these
+    are:
+
+    * ``COMPUTE_NET_ATTACH_INTERFACE``
+    * ``COMPUTE_DEVICE_TAGGING``
+    * ``COMPUTE_NET_ATTACH_INTERFACE_WITH_TAG``
+    * ``COMPUTE_VOLUME_ATTACH_WITH_TAG``
+    * ``COMPUTE_VOLUME_EXTEND``
+    * ``COMPUTE_VOLUME_MULTI_ATTACH``
+    * ``COMPUTE_TRUSTED_CERTS``
+
+    Any traits provided by the driver will be automatically added
+    during startup or a periodic update of a compute node.  Similarly
+    any traits later retracted by the driver will be automatically
+    removed.
+
+    However any traits which are removed by the admin from the compute
+    node resource provider via the Placement API will not be
+    reinstated until the compute service's provider cache is reset.
+    This can be triggered via a ``SIGHUP``.