OSDMonitor: add 'osd pg-temp ...' command

OSDMonitor: add 'osd pg-temp ...' command

ceph osd pg-temp <pgid> [<osd1> [<osd2> ...]]

Examples:

ceph osd pg-temp 0.2 0 1 2 # set pg_temp mapping for 0.2 to osds [0,1,2]
ceph osd pg-temp 0.2 3     # set pg_temp mapping for 0.2 to osds [3]
ceph osd pg-temp 0.2       # remove pg_temp mapping for 0.2

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>