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

This bug was originally filed in Launchpad as [LP: #1871858](https://bugs.launchpad.net/cloud-init/+bug/1871858)

Launchpad details

affected_projects = []

assignee = None

assignee_name = None

date_closed = None

date_created = 2020-04-09T14:35:56.995083+00:00

date_fix_committed = None

date_fix_released = None

id = 1871858

importance = wishlist

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1871858

milestone = None

owner = oddbloke

owner_name = Dan Watkins

private = False

status = triaged

submitter = oddbloke

submitter_name = Dan Watkins

tags = []

duplicates = []

_Launchpad user **Dan Watkins(oddbloke)** wrote on 2020-04-09T14:35:56.995083+00:00_

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