fix #912: centos/redhat confusion

fix #912: centos/redhat confusion

With the introduction of unmanaged containers a check was
added to verify that the ostype is the same as the
autodetected one.
Since our CentOS plugin was named 'redhat' but the ostype
setting was 'centos' this error triggered and rendered
centos containers unusable.

Renamed the LXC::Setup::Redhat package LXC::Setup::CentOS
and changed the plugin name to and autodetected type from
'redhat' to 'centos' as well.