detect APT_PROXY from host apt.conf

Introduce a new HTTP_PROXY variable in /etc/default/lxc.  If unset or
set to none, then behavior continues as before.  If set to 'apt', then
any http::proxy set in apt.conf will be used as http_proxy for
debootstrap, and specified in the container's
/etc/apt/apt.conf.d/70proxy.  If set to something else, then the
value of HTTP_PROXY will be used as http_proxy for debootstrap and
specified in the container's 70proxy.

Changelog: (apr 23) merge the two apt proxy detection functions.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>