Collect tox logs from job runs

Copy any files from .tox/*/logs into a tox subdirectory of the job
log artifact URL. This should enable deeper debugging of the
occasional failures we see which appear to be defects in tox or
within the system impacting its ability to start the test runner.

Change-Id: Id8d5d74d140436f4b05740cbd98249aef7ae51d4
Related-Bug: #1282795
Merge "Collect tox logs from job runs"