nginx: add --nginx-sleep-seconds (#8163)

* nginx: add --nginx-sleep-seconds

As described in #7422, reloading nginx is an asynchronous process and
Certbot does not know when it is complete. In an environment where this
reload takes a long time, the nginx plugin suffers from an issue where
it responds to and fails the ACME challenge before the nginx server is
ready to serve it.

Following the discussion in a previous PR #7740, this commit introduces
a new flag, --nginx-sleep-seconds, which may be used to increase the
duration that Certbot will wait for nginx to reload, from its previously
hard-coded value of 1s.

Fixes #7422

* update CHANGELOG

* nginx: update docstring for nginx_restart

certbot/CHANGELOG.md

@@ -9,6 +9,7 @@ Certbot adheres to [Semantic Versioning](https://semver.org/).
 * Third-party plugins can be used without prefix (`plugin_name` instead of `dist_name:plugin_name`):
   this concerns the plugin name, CLI flags, and keys in credential files.
   The prefixed form is still supported but is deprecated, and will be removed in a future release.
+* Added `--nginx-sleep-seconds` (default `1`) for environments where nginx takes a long time to reload.
 
 ### Changed