integration_tests: add test for LP: #1898997

[OddBloke]

Proposed Commit Message

integration_tests: add test for LP: #1898997

This introduces the `lxd_config_dict` mark, used to specify a
free-form configuration dict to LXD for tests which only run there; and
the `not_xenial` and `not_bionic` marks, used to skip tests on
xenial/bionic via a basic release skipping mechanism.

(The `lxd_config_dict` change further complicated `_client`, so a minor
refactoring is applied.)

Additional Context

This is WIP because it needs canonical/pycloudlib#66 to land for network config to LXD VMs to work.

Test Steps

Run CLOUD_INIT_PLATFORM=lxd_vm pytest --log-cli-level=INFO tests/integration_tests/bugs/test_lp1898997.py to see a failure.

Run CLOUD_INIT_PLATFORM=lxd_vm CLOUD_INIT_CLOUD_INIT_SOURCE=PROPOSED pytest --log-cli-level=INFO tests/integration_tests/bugs/test_lp1898997.py to see a pass.

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

[OddBloke]

This is now ready for review; it still needs canonical/pycloudlib#66 to land before it can land (so I've left the wip tag) but I wouldn't anticipate any review changes to that PR would lead to changes here (because, essentially, this is exercising the existing pycloudlib API which was found to have an internal bug).

[OddBloke]

This is now ready for review (and subsequent landing, once our Travis issues are sorted).

[TheRealFalcon]

Some minor questions/nits, but nothing that should prevent merging. Thanks for all the LXD yak shaving!

[TheRealFalcon]

Jk...just remembered something. You should update the commit hash of pycloudlib to the latest commit in integration-requirements.txt. I also forgot to do that in my last PR.

[OddBloke]

Jk...just remembered something. You should update the commit hash of pycloudlib to the latest commit in integration-requirements.txt. I also forgot to do that in my last PR.

Ooh, good catch. I wonder if we should update the test steps to instruct people to manually run the integration tests using tox; that would catch such issues, I think.