Replace os.system call in tarfile creation with subprocess function

[shreyb]

As mentioned in #546, we should start making the code more robust by replacing os.system calls like the one in tarfiles.tar_up() with either call or run from the subprocess module.

So for this particular case:

• Make the appropriate os.system -> subprocess.run/call substitution in tarfiles.tar_up()
• There are a few notes in tarfiles.py as to what needs to change with the error handling after the previous item is done. Make those changes.
• Add to unit tests to make sure this method is still acting correctly
• Change integration tests in test_full_home.py to observe new error handling