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