Add default NTP configuration to ISO image cloud-init

[eak13]

As a Image Builder user, I want to have explicit default NTP server configuration specified in the ISO cloud-init, so that I am able to override the NTP settings downstream when generating the ISO image.

Currently the NTP server is not explicitly set, so the Ubuntu default configuration is used. This prevents users from overriding the NTP server information with the own values when generating the ISO image.

Update networking data to add default NTP server entries. Update the ansible task to set the NTP server configuration based on the declared list of NTP servers.

[craiganderson]

"Nice to have" story, as I don't see a reason we need precise time-keeping on a node that's only around for a few minutes during deployment.

However it should be fairly easy to do. Refer to the cloud-init updates used to configure NTP on target nodes, and include them in the ephemeral ISO secret in airshipctl manifests. You will need to change it to a string blob because that's how the ephemeral ISO cloud-init user-data is represented, and perform replacements of NTP servers using kustomize regex instead of specific data fields in YAML.