SIGUSR2 logs stack traces for active threads.

* doc/source/zuul.rst: Document SIGUSR2 behavior.

* zuul/cmd/server.py: When SIGUSR2 is received log stack traces for all
active running threads. This is useful for debugging deadlock
situations. Note that this makes use of sys._current_frames which may
not play nice with all implementations of Python.

*tests/test_stack_dump.py: Test the stack dump signal handler with a new
test file, class, and test method.

* requirements.txt: Add argparse to requirements list so that py26 tests
can pass when zuul.cmd.server is imported.

Change-Id: I8ad8155b16f324e832c191f0a619ff89ef804a87