cephadm: create functional mock for fchown

cephadm: create functional mock for fchown

The pyfakefs library apparently doesn't have its own mock for os.fchown.
This means that code using fchown currently calls into a mock with
no affect on the fake fs. For some reason I don't fully understand,
existing test cases work because they don't always follow the pattern
of open-write-rename. Switching to `write_new`, which always does a
rename, breaks some of the assertions performed in the tests on the fake
fs. Add a mock fchown that updates the state of the fake fs so
that converting call sites to use `write_new` will continue to work.

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