Hot fix for failing video downloads on OSX.

[cpauya]

Hi @aronasorman, this PR does the following:

• fixes Cannot connect to server for downloading video files from current development installs #3704
• fixes Settings object has no attribute "DO_NOT_RELOAD_CONTENT_CACHE_AT_STARTUP" #4114
• fixes For Mac OS, Video Downloading Issue #3990

As per learningequality/ka-lite-installers#100 - PyRun doesn't include the multiprocessing module by default. So we check if call_command() is being run inside PyRun and call another method that uses the subprocess module.

If not running on Pyrun, we resume with current functionality that calls the CommandProcess class which uses the multiprocessing module.

To test this with Pyrun

1. Checkout the installers repo on another directory, then run setup.sh of the OSX installers repo.
2. Press Ctrl+C or cancel when it's already downloading the "ka-lite" repo, we only need the downloaded PyRun package.
3. Checkout this PR's branch.
4. Run "<installers-working-dir>/installers/osx/temp/pyrun-2.7/bin/pip" install . to install the ka-lite-static module.
5. Run "<working-dir>/installers/osx/temp/pyrun-2.7/bin/pip" freeze - to see double-check that ka-lite-static is installed.
6. Run launchctl setenv KALITE_PYTHON "<bin/pyrun full path>". Example: launchctl setenv KALITE_PYTHON "/Users/cyril/w/fle/ka-lite/installers/osx/temp/pyrun-2.7/bin/pyrun"
7. Restart the terminal.
8. Check that we have a KALITE_PYTHON env var with env | grep KA.
9. You can now run bin/kalite manage setup command and test the device registration, video downloads, language downloads, and etc to see if they work.

To test with the OSX default Python, just unset the KALITE_PYTHON with launchctl unsetenv KALITE_PYTHON and restart the server.

Tips

1. Make an alias of the 'pyrun' and 'pip' to ease typing.
2. Set DEBUG=True in your local_settings.py for easier tracing.

Other fix
This also sets the default value for the DO_NOT_RELOAD_CONTENT_CACHE_AT_STARTUP = False at kalite/settings/__init__.py to fix crashes when settings.DO_NOT_RELOAD_CONTENT_CACHE_AT_STARTUP is used in other Python codes as per #4114. I had to do this to continue testing the fixes above.

[cpauya]

Hi @benjaoming - yes we are still using local_settings.py on OSX. I tried using bin/kalite start --settings kalite.my_settings but couldn't remember why I can't make it work.

But we are leaving local_settings.py as soon as 0.14.x is released - had to deal with more important tasks. :)

I saw your merged PR already and will use that. Thanks.

[benjaoming]

I think it's about time we merge this so we don't have stuff critical lying around unmerged just before launch!

In a more general setting: Trying to import a module or an object from a module and catching ImportError is normally a better pattern for these cases.

But this python-packages/fle_utils/django_utils/command.py is terrible and should go, I opened #4139 for this.

[cpauya]

Thanks @benjaoming - looking forward for the refactor at #4139 later. :)