ceph: flush stdout on watch print

Signed-off-by: Sage Weil <sage@inktank.com>
qa/workunits/cephtool/test.sh: look for 'ceph log' via -w, not in log file

'ceph-conf ...' doesn't give you final/default values, only what is in the
conf file.  Use -w output to test this instead.

Fixes: #5327
Signed-off-by: Sage Weil <sage@inktank.com>