Fix NTP server configuration

This fix is to make sure the list of NTP servers stored in session
reflects the reality of webui, i.e. order of servers entered + blank
fields. The "empty" servers are then removed from the list just
before the configuration is to be saved (commited into DB).

https://bugzilla.redhat.com/show_bug.cgi?id=1153633
Merge pull request #897 from mzazrivec/fix_ntp_server_configuration

Fix NTP server configuration