integration_tests: log image serial if available

[OddBloke]

Proposed Commit Message

integration_tests: log image serial if available

Ubuntu cloud images ship /etc/cloud/build.info which includes a line
with the build serial used to identify the image:

    serial: 20210108

This is valuable information when verifying Ubuntu issues (to confirm
that testing is happening against the expected image), but is also
useful when debugging test failures: manifests of all packages in (the
base) images can be found at http://cloud-images.ubuntu.com/

Test Steps

Run a test (against Ubuntu) and observe:

2021-01-13 17:52:03 INFO      integration_testing:clouds.py:178 image serial: 20210112

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