ipaclient_setup_ntp: Use time sync code from 4.6.4 if sync_time is not defined

The FreeIPA versions since 4.7.0 are using chrony and also the new
sync_time function for time synchronization which has been added to
ipaclient/install/client.py.

The old version in ipaclient_setup_ntp has been updated to the code that
has been used in 4.6.4.