ceph::conf: add support for 'mon osd (near)full ratio'

Added new optional parameter to ceph::conf:
- mon_osd_full_ratio
- mon_osd_nearfull_ratio

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
rspec: fix typo in describe/context

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