Fix QA and documentation about no initrd case

In Ganeti 1.2, “none” was used to signify no initrd. In 2.0 we have
changed to “no_” as a prefix (i.e. “-H no_initrd_path”) and thus we
document in the manpage this.

The QA suite is changed accordingly.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>