custom ntp server in phosphor-networkd

[Alexander A. Filippov]

Our customers complain that they can't set custom ntp while dhcp is enabled on
bmc network interface.

I found out that the phosphor-networkd doesn't allow it in fact and it awaits
the list of ntp-servers in response from dhcp server. When BMC is configured
with a static IP address phosphor-networkd keep a list of ntp-service in the
configuration file of the network interface. In my opinion it is wrong.

I propose to change this behavior: 
 - the list of ntp-service should be kept in /etc/systemd/timesyncd.conf
 - the customization of ntp-servers should be independent from the network
   inferface configuration.

thoughts, doubts?

Regards,
Alexander