Permit use of wheel mirrors in pypi element.

pypi-mirror creates a separate mirror index for wheels (one per OS
that mirrors are built on). To be able to use it one then needs to be
able to export multiple mirrors for inclusion in pip.conf. As a drive
by I made it possible to disable the use of the pypi.python.org index
without using --offline (as --offline has larger impact).

Change-Id: I3e85a8069b18cafd7eae4cd0591821acc3b5a739