cephadm: add write_new function for robust file writes

cephadm: add write_new function for robust file writes

The cephadm code has a very common pattern made of at least one of
the three following steps:
* call fchown on the open file to set ownership
* call fchmod on the open file to set permissions
* rename the file from a temp name to final name

Add the write_new function to encapsulate these common actions.
If owner is not None then fchown will be called.
If perms is not None then fchmod will be called.
An optional encoding value may be passed.
It always uses a temporary file as a temporary file ensures that
there can never be a partially written file even in the event of
a power outage or system crash.
Encapsulating this all into a function also allows us to make
changes to this approach in the future without touching every
call site using `open(..., "w")` etc.

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit 7a8bfb91af246e36242aee90c3f9245fcdf6a318)