centos template: prevent mingetty from calling vhangup(2)

When using unprivileged containers, tty fails because of vhangup. Adding --nohangup to mingetty fixes the issue. This is the same problem occurred for oracle template, commit 2e83f7201c5d402478b9849f0a85c62d5b9f1589

Signed-off-by: Claudio Alarcon <clalarco@gmail.com>