rbd: add options, cleanup setup/init functions

Added options for RBD name, ceph pool and ceph(x) client name instead
of hard coding.

Cleaned up fio_rbd_init() and fio_rbd_setup(), reuse rbd_data via
thread_data between the functions.

Reuse _fio_rbd_disconnect() and fio_rbd_cleanup() in setup case instead
of using duplicated code.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>

(cherry picked from commit cace4538f2ffbbb1405999f72ee67e7a9515c6d6
git@github.com:dalgaaf/fio.git)
fio.1: add rbd options to man page

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>

(cherry picked from commit 6e44a82a724b7ad0a559c22eec694e8afa7d2a20
git@github.com:dalgaaf/fio.git)
Merge pull request #1 from TelekomCloud/wip-da-rbd-engine

Add options, man page and cleanup