Don't ignore secondary node silently

Currently on non-mirrored disk templates the secondary node is ignored
silently. This patch adds a check for this case, and warns the user
should this be happening. This solves issue 113.

The patch also moves a prereq check to an argument check. This is ok
because the iallocator framework already checks it has a secondary node,
when it needs one.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>