use nslookup or curl instead of ping in check-host.sh

[mlasak]

in some environments icmp is not routed and so ping might not work. however, host is reachable.
suggesting to use DNS lookup in case domain name was specifed or use curl to a known ressource on HTTP(s) port

[FritzHeiden]

Neither nslookup nor curl are suitable in this case: we need to check if IP is reachable, nslookup doesn't resolve local network IPs and curl requires a server to respond, which is not running at the moment the script runs.

To avoid blocking the server startup even though it is reachable the check-host.sh script now reports a warning which lets the server startup continue. #107