known_hosts: support --diff (#20349)

* known_hosts: support --diff

* known_hosts: support --diff also without --check

* Add unit tests and fix incorrect diff in one corner case

Tests are good!

* Refactor for readability

* Python 3 compat

* More Python 3 compat

* Add an integration test for known_hosts

* Handle ssh-keygen -HF returning non-zero exit code

AFAICT this is a bug in ssh-keygen in some newer OpenSSH versions
(>= 6.4 probably; see commit dd9d5cc670):
when you invoke ssh-keygen with -H and -F <host> options, it always
returns exit code 1.  This is because in ssh-keygen.c there's a function
do_known_hosts() which calls

  exit (find_host && !ctx.found_key);

at the end, and find_host is 1 (because we passed -F on the command line),
but ctx.found_key is always 0.  Why is found_key always 0?  Because the
callback passed to hostkeys_foreach(), which is known_hosts_hash(),
never bothers to set found_key to 1.

* This test does not need root

* Avoid ssh-ed25519 keys in sample known_hosts file

Older versions of OpenSSH do not like them and ssh-keygen -HF
aborts with an error when it sees such keys:

  line 5 invalid key: example.net...
  /root/ansible_testing/known_hosts is not a valid known_hosts file.

* Fix Python 3 errors

Specifically, the default mode of tempfile.NamedTemporaryFile is 'w+b',
which means Python 3 wants us to write bytes objects to it -- but the
keys we have are all unicode strings.

test/integration/targets/known_hosts/meta/main.yml

@@ -0,0 +1,2 @@
+dependencies:
+  - prepare_tests