HRoller: print only online nodes

To make the graphs work even when instances live on offline nodes (eg.
because we're offlining them just to exclude them, or because they have
instance still on them) we just filter them out at the end, when we're
going to print out the result.

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