The pypi element is cool, but some folk have local network mirrors
which we should permit them to use.
Change-Id: Ie840ad1184e72b0e01966eee0298cfd6511b6c19
Just adding an element shouldn't make a disk image build crash, even if
we're not using the element for its intended purpose.
Change-Id: I2ec91ce4975b3b0deee7c85bb223f854c694553f
Using a custom pypi mirror can be very convenient, making image builds
substantially faster - because we create multiple virtual
environments we benefit more than single-virtualenv users would.
Change-Id: I997daf1f9477c447e1fb30818aea9e80a49b31a6
