support ssh configuration logic through sshd_config.d on google backend

[sergiocazzolato]

Some images (like ubuntu kinetic) override the ssh configuration by using the new sshd_config.d directory and no longer ship a /etc/sshd_config file.

This change makes sure these systems will have the needed shell configuration to run spread.

As the first value wins, we need to use 00- for the configuration file to make sure this is the first file with that config

[mardy]

Can this branch fix https://bugs.launchpad.net/ubuntu/+source/snapd/+bug/1991745 ?

[sergiocazzolato]

Updated the change.

[ZeyadYasser]

@sergiocazzolato could you please also extend the fix to the LXD backend?

~~It might be a good idea to extend the LXD spread test to run with other system variants such as ubuntu-22.04.~~ EDIT: This is already being addressed in https://github.com/snapcore/spread/pull/182 https://github.com/snapcore/spread/blob/ded9133cdbceaf01f8a1c9decf6ff9ea56e194d6/tests/lxd/task.yaml#L12-L13 We could also extend the base spread.yaml in a follow up PR to add multiple systems most importantly 22.04 to test the google backend. https://github.com/snapcore/spread/blob/ded9133cdbceaf01f8a1c9decf6ff9ea56e194d6/spread.yaml#L17-L31