ceph.in: write bytes to stdout in raw_write()

ceph.in: write bytes to stdout in raw_write()

in python3, sys.stdout.buffer is an io.BufferedWriter, while in python2
`sys.__stdout__` is a plain file. the former only accepts "bytes". so if
we send it a "str", it complains:

TypeError: a bytes-like object is required, not 'str'

it happens when quitting from the interactive mode of ceph CLI. in that
case, `new_style_command()` returns a tuple of `0, '\n', ''`, where the
second element is a str.

in this change, we always send `bytes` to raw_stdout.

Signed-off-by: Kefu Chai <kchai@redhat.com>