Fix min config value for shutdown_timeout option

The shutdown_timeout config option was added in commit
c07ed15415 without a min
value. The min value was later added in commit
d67ea6e554 and set to 1,
which means the option can never be configured to mean
"always immediately shutdown". That is also inconsistent
with the description of the "os_shutdown_timeout" image
property in the glance metadata definition which says the
value can be set to 0 to force an immediate shutdown of
the guest.

This fixes the min value in the config option to be 0 which
is already what happens if we are not performing a clean
shutdown at all.

Change-Id: I399b9031d2aa477194697390e2cd3f78e3ac0f91
Closes-Bug: #1799707

nova/conf/compute.py

@@ -1029,7 +1029,7 @@ Possible values:
 """),
     cfg.IntOpt("shutdown_timeout",
         default=60,
-        min=1,
+        min=0,
         help="""
 Total time to wait in seconds for an instance to perform a clean
 shutdown.
@@ -1038,7 +1038,8 @@ It determines the overall period (in seconds) a VM is allowed to
 perform a clean shutdown. While performing stop, rescue and shelve,
 rebuild operations, configuring this option gives the VM a chance
 to perform a controlled shutdown before the instance is powered off.
-The default timeout is 60 seconds.
+The default timeout is 60 seconds. A value of 0 (zero) means the guest
+will be powered off immediately with no opportunity for guest OS clean-up.
 
 The timeout value can be overridden on a per image basis by means
 of os_shutdown_timeout that is an image metadata setting allowing
@@ -1047,7 +1048,7 @@ need to shut down cleanly.
 
 Possible values:
 
-* Any positive integer in seconds (default value is 60).
+* A positive integer or 0 (default value is 60).
 """)
 ]
 

releasenotes/notes/bug-1799707-fix-shutdown_timeout-min-22ce0b373af1ec90.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    The ``[DEFAULT]/shutdown_timeout`` configuration option minimum value has
+    been fixed to be 0 rather than 1 to align with the corresponding
+    ``os_shutdown_timeout`` image property. See bug
+    https://launchpad.net/bugs/1799707 for details.