CERTTF-448 Agent changes to receive logs from Conserver instance

[val500]

Description

This PR gives the agent the ability to get serial console output from a Conserver server, post it to the Testflinger server, and save it in a artifact. This required several changes:

1. New field in the agent configuration file for the conserver server IP address
2. Changes to the CommandRunner class
   • The run function was split up into a couple different functions:
     • create_command_thread: Creates and starts the thread to run the command and stores it as a instance variable
     • monitor_command_output: Checks the stop conditions and posts command output to the handlers
   • run_async function was added
     • Since run blocks the main thread during the duration of the command run, it is unsuitable for continuously streaming output from conserver.
     • This new function creates a new command thread and runs the monitor_command_output function as a separate thread.
   • kill_async function was added
     • This function kills the running process and joins the two running threads
   • Handling of termination signal moved to job.py
     • Since there are two runners now, we need to cleanup both runners on termination signal. The new function in job.py, cleanup_runners, is ran on termination signal now.
3. Creates new handlers to differentiate between live device output and live serial output.

Resolved issues

Resolves https://warthogs.atlassian.net/browse/CERTTF-448

Documentation

Web service API changes

N/A

Tests

Unit tests were added to test_serial.py

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