Add external DNS and NTP support

[Oleksiy Molchanov]

Add support of external DNS and NTP.

Add support of using external DNS and NTP in HA mode.
Change-Id: I9d741c638c1bd034ec3b451cffa1e70a3fee802a
Implements: blueprint external-dns-ntp-support
Closes-Bug: 1368514

https://review.openstack.org/#/c/130827/67

[Irina Povolotskaya (ipovolotskaya)]

Sam already started working on it within https://review.openstack.org/#/c/153839/
Sam, could you take up the bp then?

[Sam Stoelinga (sammiestoel)]

I started working on it before when I didn't know about the BP. I only changed the documentation on how to change upstream DNS server for Fuel 6.0 and before. So please discard my previous work and re-assign to somebody else who understands the new changes better than me.

[Oleksiy Molchanov (omolchanov)]

Please add to documentation that when changing upstream ntp servers per cluster we cannot be sure that deployment will succeed.

Ideally NTP servers on master should be equal to upstream NTP servers per environments.

[Stanislaw Bogatkin (sbogatkin)]

I think we should write that at deploy time on master node must be equal to time on cluster nodes. Most easy way to do it - sync master and cluster with same upstream NTP server, that's all.