use openntpd for genesis host

[craiganderson]

Update documentation to use openntpd instead of ntpd since the former uses ephemeral ports for outgoing connections.

[craiganderson]

Is there any reason we wouldn't use the same ntp agent on genesis as we use on the other bare metal deployed by MaaS (if opentntpd is not already what is deployed)? openntpd has some limitations like for leap second support which me may need to look more closely at. Some of the earlier versions had accurate issues, didn't track clock drift, or relied on BSD kernel features - maybe all resolved by now.

@sh8121att Could you describe the incident that initiated this change request?
Thanks