use args.ceph_conf if passed in

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
add the flag to specify for ceph-conf

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
error if users calls 'new' with custom ceph conf flag

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
Merge pull request #234 from alfredodeza/9136

allow to specify a ceph conf file