1. Proposed title of this feature request
--> Agent Based Installer: Allow Replacing default NTP Servers in agent-config.yaml 

2. What is the nature and description of the request?
--> There should be a option to configure NTP during the UPI installation process.
    If the environment is in a disconnected setting there should be option to exclusively use own NTP servers, rather than just adding additional ones.
    Retaining the old/default NTP servers in the configuration does not make sense in disconnected environment.

3. Why it is needed? (List the business requirements here)
--> It would ensure that the hosts exclusively use corporate NTP systems and do not have the old/unused NTP systems configured. By using the current available "additionalNTPSources" property, the old NTP servers are still available but marked and unreachable NTP sources (chronyc sources). This could lead to the systems receiving an incorrect time or querying the incorrect NTP servers for a date. (Not tested, but most likely) unnecessary alerts will get triggered, because chronyd expect to reach all NTP servers and not only the additional configured ones.

4. List any affected packages or components.
--> Agent Based Installer (agent-config.yaml), OpenShift4