rsync: ping before trying ssh

Change d6fed34 modified the archive
reachable check to use ssh instead of ping. This is much more resource
intensive though, so change it to try a ping first, and ssh only if the
ping fails.

run/rsync_archive/archive-is-reachable.sh

@@ -2,4 +2,4 @@
 
 ARCHIVE_HOST_NAME="$1"
 
-ssh -q -o ConnectTimeout=1 "$RSYNC_USER"@"$ARCHIVE_HOST_NAME" exit
+ping -q -w 1 -c 1 "$ARCHIVE_HOST_NAME" &> /dev/null || ssh -q -o ConnectTimeout=1 "$RSYNC_USER"@"$ARCHIVE_HOST_NAME" exit