Update upload_file code

[ssardina]

I think we should not remove this facility as some users may not have sufficient hosting space.

But we can update the source for this update_file function in contest_runner.py:

• [x] use format strings
• [x] replace OS calls os.system("rm %s" % file_full_path) with Python based libs
• [x] replace os.system with subprocess.

[ssardina]

Have done several formatted strings but we can keep doing that incrementally.

The os. calls could remain like that because we really do not need the output of those proceses.

Finally, we should do tar outside, not in Python, because it is much faster (written in C, rather than in Python).

So, we can close this issue now..