zuul: no fetch from pypi and drop statsd dependency

`python setup.py install` will happilly fetch dependencies from the
pypi repository which we do not want.  Instead we provide all the
dependencies via Debian packages and want to skip downloading entirely.

`easy_install` comes with an option to filter out the hosts we can
download from. Setting --allow-hosts=None will forbid downloading.

Since we do not package the `statsd` python module, we hack Zuul list of
requirement to comment it out.  That will let easy_install proceed the
installation.

Change-Id: I2571fa8f5630c1200e6c265bef770ae7ec167fe5