Use ansible_mitogen plugin to speedup deploys

mitogen speeds up an ansible run, according to the docs:
" Expect a 1.25x - 7x speedup and a CPU usage reduction of at least 2x, …"
The problem is that we need to ship a bundled version of it since atm
you need to provide the path to the python module, which varies according
to the way how ppl install it (venv, pip user vs. system installation etc).
See https://github.com/dw/mitogen/issues/568 for details.