Support for custom NTP during deployment

[dominikzorgnotti]

Please add the ability to (optionally) specify custom NTP servers during OVF deployment with an additional parameter.
Currently, the appliances gets deployed with the default photon settings which have a fallback to the google NTP servers as documented per KB 76088.

For environments where the internet traffic is blocked by default this is not feasible and requires manual effort to change the NTP settings as a day2 operation after deployment.