cloud-init should support parsing ssh_config/sshd_config files with Include directives

[Dan Watkins]

In OpenSSH 8.2[0], support for Include directives in ssh_config and sshd_config was introduced. In Debian/Ubuntu version 1:8.2p1-1 [1], Include directives were added to the config files shipped in the package (and in 1:8.2p1-4, the directories themselves were added).

cloud-init should handle includes in its SSH configuration parsing, so that images mastered with configuration in the .d directories will behave consistently with other images.

[0] https://www.openssh.com/txt/release-8.2
[1] https://launchpad.net/ubuntu/+source/openssh/1:8.2p1-1
[2] https://launchpad.net/ubuntu/+source/openssh/1:8.2p1-4

[Dan Watkins (oddbloke)]

For reference: as noted in https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/1873528, the way in which OpenSSH processes includes is unlike other pieces of software; we should bear that in mind when implementing this.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3645