Move some common Ubuntu config

This introduces a new /usr/share/lxc/config directory containing common
configuration snippets.

The two Ubuntu templates are then simplified to just include the
relevant entries avoiding a whole lot of hardcoded cgroup, capabilities
and mount points configuration.

An extra comment is also added at the top of all generated configuration
files telling the user to look at lxc.conf(5) for more information.

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