Fix the HTTP code for reaching max_concurrent_deploy: 503 instead of 500

Change-Id: I3d8c7724c1d44baa67a6364dde2f52abdb906526

ironic/common/exception.py

@@ -857,7 +857,7 @@ class ImageRefIsARedirect(IronicException):
             redirect_url=redirect_url)
 
 
-class ConcurrentActionLimit(IronicException):
+class ConcurrentActionLimit(TemporaryFailure):
     # NOTE(TheJulia): We explicitly don't report the concurrent
     # action limit configuration value as a security guard since
     # if informed of the limit, an attacker can tailor their attack.

releasenotes/notes/max_concurrent_deploy-7a31ba142bf5ad5c.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    When Ironic hits the limit on the number of the concurrent deploys
+    (specified in the ``[conductor]max_concurrent_deploy`` option), the
+    resulting HTTP code is now 503 instead of the more generic 500.