lxc-oracle: mount cgroup:mixed

- Mounting cgroup:mixed prevents systemd inside the container from
  moving its children out of the cgroups lxc setup. This ensure the
  limits setup in the configuration or with lxc-cgroup are effective.

- Update for the OL7 channel name that will be used on
  public-yum.oracle.com.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>