Behaviour for "default NTP source" needs at least some clarifying

[ydirson]

v10.10.7 changes the way NTP is handled to cope with new support for getting NTP servers from DHCP. This is real good news.

However, the new source="default" behavior seems strange in several ways:

• the documentation does not document it (what does "use default NTP servers" as alternative to well-defined dhcp|manual|none choices?) Likely the "the default shall be..." sentence is meant to explain that, except that it does not explicitly states it applies to default, which is likely not explicitly documented as being the default value if the ntp element is not provided.
• "Use default NTP servers" in the TUI is likely to cause confusion; what would be the reason not to select an actual source? Its effect is even not clear at all from the code.
• the default-selection log exists in answerfile.py but leaves a branch where results['ntp-config-method'] = "default", which should likely not be