mon/PGMap: pretty output for "pg dump osds" command

mon/PGMap: pretty output for "pg dump osds" command

    before:
    [root@c7 yum.repos.d]# ceph pg dump osds
    dumped osds in format plain
    osdstat kbused  kbavail kb      hb in   hb out
    1       12914796        88268416        101183212       [0]     []
    0       12914796        88268416        101183212       [1]     []
     sum    25829592        176536832       202366424

    after:
    [root@c7 yum.repos.d]# ceph pg dump osds
    dumped osds in format plain
    osdstat used    avail   total   hb in   hb out
    1       71883 kB        125 MB  195 MB  [0]     []
    0       71883 kB        125 MB  195 MB  [1]     []
     sum    140 MB  250 MB  390 MB

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>