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>
Add support for new create=(dir,file) mount option

Just like we already had "optional", this adds two new LXC-specific
mount flags:
 - create=dir (will do a mkdir_p on the path)
 - create=file (will do a mkdir_p on the dirname + a fopen on the path)

This was motivated by some of the needed bind-mounts for the
unprivileged containers.

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