cephadm: convert some functions to use write_new

cephadm: convert some functions to use write_new

Convert a lot of the basic uses of the pattern:
with open(...) as f:
  f.write(...)
  os.fchown(f, ...)  # sometimes
  os.fchmod(f, ...)  # sometimes
  os.rename(...)   # sometimes

These are the most obvious cases to convert to `write_new`
and should largely be uncontroversial.

Signed-off-by: John Mulligan <jmulligan@redhat.com>