integration_tests: introduce lxd_use_exec mark

[OddBloke]

Proposed Commit Message

integration_tests: introduce lxd_use_exec mark

pycloudlib has modified the way LXD executes tests
(https://github.com/canonical/pycloudlib/pull/114): it will always use
SSH to access them by default, instead of using `lxc exec`.  This
behaviour is transparent for the majority of cloud-init's integration
tests, but some currently depend on using `lxc exec` to access instances
with (intentionally) broken networking: obviously these are not
accessible via SSH.

pycloudlib retains support for switching an instance to use `lxc exec`.
This commit introduces the `lxd_use_exec` mark, which tests can use to
indicate to the integration testing framework that they should be so
switched, and applies it to all applicable tests.

Additional Context

See canonical/pycloudlib#114

Test Steps

CI tests will indicate that LXD containers work, a manual run is required for LXD VMs.

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[blackboxsw]

As written this assumes the a test will always interact using lxc exec instead of ssh. Do you see any use-cases that we want to plan for where we really only want lxc exec for some commands and ssh for others?

[OddBloke]

Do you see any use-cases that we want to plan for where we really only want lxc exec for some commands and ssh for others?

None immediately present themselves to me. Such cases can be handled today, something like:

@pytest.mark.lxc_use_exec
def test_example(client):
    client.execute("unbreak-ssh-access-somehow")
    client.instance.execute_via_ssh = True
    client.execute("echo ssh works")

If we see more than one or two such cases, we should look at improving the API.

[blackboxsw]

Thanks LGTM