Add config parameter to change stop timeout during daemon shutdown

This fix tries to add a daemon config parameter `--shutdown-timeout`
that specifies the timeout value to stop containers gracefully
(before SIGKILL). The default value is 15s.

The `--shutdown-timeout` parameter is added to daemon options and
config file. It will also be updated during daemon reload.

Additional test cases have been added to cover the change.

This fix fixes #22471.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

man/dockerd.8.md

@@ -56,6 +56,7 @@ dockerd - Enable daemon mode
 [**--registry-mirror**[=*[]*]]
 [**-s**|**--storage-driver**[=*STORAGE-DRIVER*]]
 [**--selinux-enabled**]
+[**--shutdown-timeout**[=*15*]]
 [**--storage-opt**[=*[]*]]
 [**--swarm-default-advertise-addr**[=*IP|INTERFACE*]]
 [**--tls**]
@@ -246,6 +247,9 @@ output otherwise.
 **--selinux-enabled**=*true*|*false*
   Enable selinux support. Default is false.
 
+**--shutdown-timeout**=*15*
+  Set the shutdown timeout value in seconds. Default is `15`.
+
 **--storage-opt**=[]
   Set storage driver options. See STORAGE DRIVER OPTIONS.