Faults from cell DB missing in GET /servers/detail

Field  is empty in the response of API GET /servers/detail if the
instance (hence instace_faults DB entry) is in nova cell DB.
Unlike that, for API /servers/:id fault is retrieved correctly no matter
in which nova cell the instance belongs.

Closes-Bug: #1856329
Change-Id: I1726f53cfeac0a67a5dacdddda2af2cc1db0af0f
Signed-off-by: Marius Leustean <marius.leustean@sap.com>

releasenotes/notes/bug-1856329-32d7f65bf08257b3.yaml

@@ -0,0 +1,11 @@
+---
+fixes:
+  - |
+    The `bug 1856329`_ is fixed where the ``fault`` field was empty in the
+    response of ``GET /servers/detail`` when instances were located in a nova
+    cell DB. The ``InstanceList.fill_faults()`` method has been updated to use
+    ``scatter_gather_all_cells()`` to properly retrieve faults from all cells,
+    ensuring fault information is consistently returned regardless of which
+    cell contains the instance.
+
+    .. _bug 1856329: https://bugs.launchpad.net/nova/+bug/1856329