[Install] Improve installation script
[Install] Improve installation script

Two changes:
  * get rid of complicated determining of latest setuptools
    package URL and switch to usage of script ez_setup.py
  * run script ez_setup.py in any case, no matter if we have
    setuptools version lesser that 0.8 or not. As result, we
    will always have latest setuptools version in virtualenv

These changes simplify the code a lot and provide reliable
installation of latest setuptools version.

See details about usage of script ez_setup.py
at https://pypi.python.org/pypi/setuptools

Change-Id: Ia8980ad4afc22daa25258c219861e5949930d199

openstack/rally,gluke77/rally,vganapath/rally,vganapath/rally,yeming233/rally,yeming233/rally,vganapath/rally,vganapath/rally,openstack/rally,gluke77/rally,openstack/rally,openstack/rally,gluke77/rally,gluke77/rally