Add --reuse-key feature (#5901)

* Initial work on new version of --reuse-key

* Test for reuse_key

* Make lint happier

* Also test a non-dry-run reuse_key renewal

* Test --reuse-key in boulder integration test

* Better reuse-key integration testing

* Log fact that key was reused

* Test that the certificates themselves are different

* Change "oldkeypath" to "old_keypath"

* Simply appearance of new-key generation logic

* Reorganize new-key logic

* Move awk logic into TotalAndDistinctLines function

* After refactor, there's now explicit None rather than missing param

* Indicate for MyPy that key can be None

* Actually import the Optional type

* magic_typing is too magical for pylint

* Remove --no-reuse-key option

* Correct pylint test disable

certbot/cli.py

@@ -1017,6 +1017,12 @@ def prepare_and_parse_args(plugins, args, detect_defaults=False):  # pylint: dis
              "certificate already exists for the requested certificate name "
              "but does not match the requested domains, renew it now, "
              "regardless of whether it is near expiry.")
+    helpful.add(
+        "automation", "--reuse-key", dest="reuse_key",
+        action="store_true", default=flag_default("reuse_key"),
+        help="When renewing, use the same private key as the existing "
+             "certificate.")
+
     helpful.add(
         ["automation", "renew", "certonly"],
         "--allow-subset-of-names", action="store_true",