integration_tests: port lxd_bridge test from cloud_tests

[OddBloke]

Proposed Commit Message

integration_tests: port lxd_bridge test from cloud_tests

Test Steps

pytest --log-cli-level=INFO tests/integration_tests/modules/test_lxd_bridge.py fails:

>       assert "WARN" not in cloud_init_log
E       assert "WARN" not in cloud-init.log string; unexpectedly found on these lines:
E         2020-12-08 22:48:08,461 - util.py[WARNING]: Running module lxd (<module 'cloudinit.config.cc_lxd' from '/usr/lib/python3/dist-packages/cloudinit/config/cc_lxd.py'>) failed

tests/integration_tests/modules/test_lxd_bridge.py:39: AssertionError

CLOUD_INIT_CLOUD_INIT_SOURCE=PROPOSED pytest --log-cli-level=INFO tests/integration_tests/modules/test_lxd_bridge.py passes.

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