Address review comments:

- make --user-agent a general CLI option
 - get_os_info return type

letsencrypt/cli.py

@@ -893,7 +893,7 @@ def _create_subparsers(helpful):
     helpful.add_group("rollback", description="Options for reverting config changes")
     helpful.add_group("plugins", description="Plugin options")
     helpful.add(
-        "certonly", "--user-agent", type=str, default=None,
+        None, "--user-agent", type=str, default=None,
         help="Set a custom user agent string for the client. User agent strings allow "
              "the CA to collect high level statistics about success rates by OS and "
              "plugin. If you wish to hide your server OS version from the Let's "

letsencrypt/le_util.py

@@ -206,7 +206,9 @@ def safely_remove(path):
 def get_os_info():
     """
     Get Operating System type/distribution and major version
-    :returns: (`str` os_name, `str` os_version)
+
+    :returns: (os_name, os_version)
+    :rtype: `tuple` of `str`
     """
     info = platform.system_alias(
         platform.system(),