find-nearest-rsync: don't pick unreachable hosts

find-nearest-rsync doesn't check the $success retval of Net::Ping->ping, only
$rtt, which is set to 0 if the hostname could not be resolved, as with
unqualified pmtpa hostnames in eqiad and vice versa. Since it's tough to beat
an RTT of 0, this bug causes find-nearest-rsync to always choose whichever
hostname it couldn't resolve. The solution is to check $success.

I am not sure how this ever worked. I guess somewhere in scap there is logic
to default to tin.eqiad.wmnet.

Change-Id: Iafe5a1e2798156e0c576b86d0833ceb89e93bc00