ceph.in: read bytes from stdin

ceph.in: read bytes from stdin

the data read from stdin is used as the input parameter for calls like
`cluster.osd_command(...)` and `cluster.mon_command`. all of them
expect a bytes `inbuf`. in Python2, this sys.stdin.read() returns a str,
and we don't differentiate str from byte back then. but we need enforce
the type now for better readablity and type correctness.

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