osd: OSDMap: have osdmap json dump print valid boolean instead of string

Fixes: 8108

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #1678 from ceph/wip-8108

osd: OSDMap: have osdmap json dump print valid boolean instead of string

Reviewed-by: Sage Weil <sage@inktank.com>