Add NTP field to BoshEnv struct

[friegger]

Similar to #133, the NTP field is missing. The missing field causes the recommended way to configure NTP as described in https://bosh.io/docs/ntp-config/#configuring-ntp-servers-in-the-director-itself to fail.

Btw the Google CPI seems to handle this generically: https://github.com/cloudfoundry/bosh-google-cpi-release/blob/f31024b544458c9032162834d91801b2eb3da1a6/src/bosh-google-cpi/registry/agent_settings.go#L71

[linux-foundation-easycla[bot]]

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• :white_check_mark: Felix Riegger (9b4ecdfabcd07ad1d9c43b86618ccbd03f75add7)

[friegger]

@xiaozhu36 Can you please review?