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: Ia7580767be2594bacbece3d7c6aae6a9aeffa56d
Closes-bug: #1351279
Merge "Don't fail tests if can't connect to NTP servers"