mon: Monitor: 'ceph df'

Fixes: #3484

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
tools: ceph: add 'ceph df' usage

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge remote-tracking branch 'gh/wip-3484'

Reviewed-by: Sage Weil <sage@inktank.com>