Add a release note for 939340

Follow-up to 939340 to add a release note about the bug-fix.

Change-Id: I202f22d40776ab5d3245b8e14021d1404a9f478d

Author: kubajj

releasenotes/notes/fix-raid-volume-name-ac9013e7b5cf3d46.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    The use of md_device as the default volume name if the volume name of a RAID
+    array hasn't been specified caused a 'Not POSIX compatible' error.
+    This has been fixed by using just the last part of the md_device.
+    Fixes https://bugs.launchpad.net/ironic-python-agent/+bug/2073406