Rework init scripts

This commit is based on the work of:
    Signed-off-by: Michael H. Warfield <mhw@WittsEnd.com>

Included changes are:
 - Bring support for lxcbr0 to all distributions
 - Share the container startup and network configuration logic across
   distributions and init systems.
 - Have all the init scripts call the helper script.
 - Support for the various different distro-specific configuration
   locations to configure lxc-net and container startup.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>