fix the bug if we set pgp_num=-1 using "ceph osd pool set data|metadata|rbd -1"
   will set the pgp_num to a hunge number.

   Signed-off-by: huangjun  <hjwsm1989@gmail.com>
Merge pull request #776 from hjwsm1989/master

   fix the bug if we set pgp_num=-1 using "ceph osd pool set data|metada...