Added templates/lxc-centos for CentOS containers.

This adds an lxc-centos template for crreating CentOS 5+ templates.  It
does NOT create CentOS 4 or earlier containers as these are way past
end of life and no longer supported.  It is based on the work of
Fajar A. Nugraha <github@fajar.net> who modified an earlier Fedora
template.  His work has been brought LARGELY into congruence with
the current Fedora template.  It still lacks the distro agnostic
bootstrap and systemd code from the Fedora template but those should
only be relevant with CentOS 7 when that can of worms pops open
sometime next year or so.

Signed-off-by: Michael H. Warfield <mhw@WittsEnd.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>