handle multiple NTP servers (bnc#895824)

sntp seems to use only the first NTP server and ignore others,
so we iterate over the list and stop waiting when we find a working one
https://bugzilla.novell.com/show_bug.cgi?id=895824
Merge pull request #345 from SUSE-Cloud/multiplentp

handle multiple NTP servers (bnc#895824)