support a custom CentOS repository

This change introduces a flag --repo to the lxc-centos template
to allow using a local repository (e.g. a loop mounted installer
iso on your web server).

Signed-off-by: Harald Dunkel <harri@afaics.de>
Acked-by: Michael H. Warfield <mhw@WittsEnd.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>