Add --reuse-key feature (certbot#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

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