[CERTTF-458] feat: use agent config in reimplemented log truncation

[boukeas]

Description

Log files are currently truncated to their last 1MB of data and that limit is hard-coded. This PR introduces a slight re-implementation of the way log files are truncated and allows the size of the truncated output log to be specified through the output_bytes agent configuration field.

The default for output_bytes when not explicitly set in the agent config is now 10MB.

Resolved issues

Resolves CERTTF-484. Note that the original task was to remove truncation altogether but making the limit parametric was deemed to be safer and more flexible.

Documentation

A paragraph was added to the section in the tutorial that discussed retrieving results. It is now explicitly stated that the results might be truncated and how the size of the truncated output log is set through the agent configuration.

Tests

Tested locally with different values for output_bytes using (variations of) this Testflinger job, which outputs random 100-character lines of output:

job_queue: myqueue
test_data:
  test_cmds: |
    for i in {1..10000}; do
        # print a random 100-character line
        tr -dc 'A-Za-z0-9!"#$%&'\''()*+,-./:;<=>?@[\]^_`{|}~' < /dev/urandom | head -c 100
        echo
    done

[val500]

LGTM

[val500]

LGTM