ceph_rest_api.py: create own default for log_file

common/config thinks the default log_file for non-daemons should be "".
Override that so that the default is /var/log/ceph/{cluster}-{name}.log
as 'usual', since ceph-rest-api is more of a daemon than a client.

Fixes: #6099
Backport: dumpling
Signed-off-by: Dan Mick <dan.mick@inktank.com>