Add distro config file /etc/lxc/lxc.conf

Add distro config file /etc/lxc/lxc.conf

This allows a distro to put the distro specific default network
configuration (for example bridge device, link type), or other lxc
configuration in the case that -f is not passed by the user to
lxc-create, in which case lxc-create will use the distro conf file as
the basis for the containers config.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>