Build PyRun to include the multiprocessing module.

[cpauya]

Branch master
Target 0.15.0
Current Behavior
Any calls to fle_utils.django_utils.command.call_command_subprocess() under OSX will use the CommandProcess class that uses the multiprocessing module.

As per eGenix's documentation on PyRun, they don't include the _multiprocessing module so calls to such will crash the ka-lite server.

Manifestations of failures include:

• video downloads fails
• language pack download fails
• calls to cron, securesync, device registrations fails

Expected Behavior
We must build PyRun to include the multiprocessing module.

Possible Solution/s
For now, use the subprocess module like the one on call_outside_command_with_output().

Related Issues
Here are the related issues affected by this:

• Cannot connect to server for downloading video files from current development installs ka-lite#3704 - Cannot connect to server for downloading video files from current development installs
• For Mac OS, Video Downloading Issue ka-lite#3990 - For Mac OS, Video Downloading Issue
• Drop Pyrun on OS X Installer and replace it with virtualenv. #98 - Dropping Pyrun on OS X Installer replace it with virtualenv

References
These references are here to confirm that multiprocessing with Django must be done carefully. They suggests to close the database connection before spawning a child process but that's not our intended solution here:

• Django multiprocessing and database connections
• Python Multiprocessing With Django

[cpauya]

As of 0.16, the issues reported here like the video and language pack downloads are already working.

Let's re-open this in case the issue pops-up again.