Code review comment for ~raharper/cloud-init:feature/update-ntp-spec

Update the commit message on the merge proposal tomake sure it is in sync
with the current code.

- I think the '_set_preferred_ntp_client' client is kind of wierd.
  could we get the same function with an attribute ?
  Then it would only be called if used. Rather than being called on init.

- 'handle' in cc_ntp is longer than i'd like. the less that function
  does the easier things are to test. but I wont insist on that.