Split the binary and source mirror defaults

Split the binary and source mirror defaults

(This was orignally reveiwed in the wrong place:
http://codereview.appspot.com/193057/show)

Many local mirrors (such as mine) only contain mirrors
of the binary packages, sufficient to create a debootstrap
chroot. Source packages, the sum of which, can be quite large and
are not typically mirrored locally. This patch allows one
to create a chroot with overrides to the desired mirrors, e.g.,

./make_chroot.sh --replace --mirror http://mirror.rtg.net/ubuntu

In this example all of the binaries come from my local mirror, all source
packages come from $DEFAULT_DEV_MIRROR.

./make_chroot.sh --replace --mirror_src http://archive.ubuntu.com/ubuntu

Here the source packages are pulled from the Ubuntu archive whereas the
binaries come from $DEFAULT_DEV_MIRROR.

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>;

[tgardner, 2010-01-22 16:50:35]

wfrichar@chromium.org: The change LGTM.