Support of an alternate pypi server

Currently pip will get the package from the https://pypi.python.org server.
For CI, it's a problem as Internet connection can be down,
the pypi server down, etc...

The usecase is for a company/user that maintain a local pypi mirror
and give the option to use this server instead of the official one

Change-Id: I83aac4646cb78827a92c9636d78238f8a6118642
Implements: blueprint support-local-pypi-server
Merge "Support of an alternate pypi server"