Block deleting compute services with in-progress migrations

This builds on I0bd63b655ad3d3d39af8d15c781ce0a45efc8e3a
which made DELETE /os-services/{service_id} fail with a 409
response if the host has instances on it. This change checks
for in-progress migrations involving the nodes on the host,
either as the source or destination nodes, and returns a 409
error response if any are found.

Failling to do this can lead to orphaned resource providers
in placement and also failing to properly confirm or revert
a pending resize or cold migration.

A release note is included for the (justified) behavior
change in the API. A new microversion should not be required
for this since admins should not have to opt out of broken
behavior.

Change-Id: I70e06c607045a1c0842f13069e51fef438012a9c
Closes-Bug: #1852610

api-ref/source/os-services.inc

@@ -349,6 +349,12 @@ Attempts to delete a ``nova-compute`` service which is still hosting instances
 will result in a 409 HTTPConflict response. The instances will need to be
 migrated or deleted before a compute service can be deleted.
 
+Similarly, attempts to delete a ``nova-compute`` service which is involved in
+in-progress migrations will result in a 409 HTTPConflict response. The
+migrations will need to be completed, for example confirming or reverting a
+resize, or the instances will need to be deleted before the compute service can
+be deleted.
+
 .. important:: Be sure to stop the actual ``nova-compute`` process on the
                physical host *before* deleting the service with this API.
                Failing to do so can lead to the running service re-creating