Don't fail tests if can't connect to NTP servers

1) In case of temporary problems with internet connection on
host server ndpd/ntpdate on Fuel master can't synchronize
time with public NTP servers and system tests fail. Actually,
it isn't necessarily because slaves sync time from master
node and cluster time still will be identical, so we should
not interrupt system tests.
2) Tests should proceed with synchronizing time on slaves
only if ntpd on master node is ready for connections.
3) If ntpdate on slaves is unable to sync time with master
node, then raise exception and break test.

Change-Id: I81f19bbd2a542d540d53ea8bdb42537b83867ee9
Closes-bug: #1351279