configure/makefile: rename default_conf to distro_conf, since it is a per-distro
default.  Then we'll be able to use the symbol LXC_DEFAULT_CONF in the code to
refer to the installed file.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>