Drop PIP_USE_MIRRORS environment variable

The PIP_USE_MIRRORS does not do anything else than adding
 the --use-mirrors option to the pip command line.

The --use-mirrors is deprecated since pip-1.5,
and does not do anything else than printing a warning message.

For using alternate pypi index url this option is not required and
not sufficient. The current way for using alternate mirrors
is to defining them in the .pip/pip.conf either manually (before
devstack), or by using the PYPI_ALTERNATIVE_URL and PYPI_OVERRIDE
environment variables.

Change-Id: Ia33e783360e5661c2ef03b77e9f7af32b2633f2f
Merge "Drop PIP_USE_MIRRORS environment variable"