Cleanup mount points automatically.

Manually listing the mount points that need cleanup is not
maintainable and makes it hard to write elements that use bind
mounting to inject resources into the build environment.

Change-Id: I7c9ade444f4ebe42552f8e321f257a7ec0a674ef
Make $HOME in the chroot be reasonable.

With sudo -E $HOME is inherited, which is nearly always wrong.

Change-Id: Ic77b9c2892c19a2fc08a07d35f76da79a346ebac
Allow using a pypi mirror to install via pip.

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