Add support multiple time&data servers

[Andrey Bychkov]

Hi, all.

I've published a draft design for the PR#2169.

https://www.freeipa.org/page/V4/NTP_Servers_Configuration

This feature allow using different time&data servers (chrony, ntpd, 
openntpd) in FreeIPA and adding new ntp services as python's class. 
Interested persons, please review the design and pull request.