zuul: switch installer from setuptools to pip

Recent version of Zuul can no more be installed with
easy_install/setuptools.  Switch to pip which fix the issue.

Setting HTTP_PROXY and HTTPS_PROXY to '.' ensure pip is not going to
fetch from pypi which is unwanted.

Resulting command:

  HTTP_PROXY=. HTTPS_PROXY=. python setup.py install

Change-Id: Ie7e4578deec0e26e28cb5fd82df9b3afdeadeb51