[celery] fixing ddtrace-run broke celery support

[alxch-]

This PR solves this issue : when patching celery with the ddtrace-run command, the import fails silently and Celery is not patched. The exception raised is 'module' object has no attribute 'argv', stemming from the way our tracer patches libraries: argv is not passed, and causes the fail celery import as the library needs it.

The PR replaces argv when it is required.

I have tested this change with a Celery app and a pymysql app, it seems to work fine.

[alxch-]

After a discussion with @SeanOC we thought about a way to test if this fix would break a CLI with arguments. I tested it with this code in a test.py file:

import sys
print(sys.argv)

And then I ran the command : ddtrace-run test.py foo bar. The result is ['test.py', 'foo', 'bar']. So it confirms that this fix does not break CLI when used with ddtrace-run, and that this change should be fine.

[SeanOC]

Nice work, looks good!