osd: OSDMap: fix output from ceph status --format=json for num_in_osds
num_up_osds returns as an int value, while num_in_osds returns as a string.
Only an int can be returned from get_num_in_osds() so for consistancy num_in_osds

Fixes: 7159

Signed-off-by: Tyler Brekke <tyler.brekke@inktank.com>