osd/OSDMap: be more aggressive when trying to balance

osd/OSDMap: be more aggressive when trying to balance

Previously we'd require the absolute deviation >= 1 before
an osd can fill in the overfull or underfull set, which as
a result can get some osds stuck severely underfull forever.

This patch tries to get us out of those corner cases mentioned
above and can be scrutinized from two aspects:
- a standard deviation is introduced to evaluate the efficiency
  of balancing, therefore making the distribution of pgs always
  converge to the perfect status (standard deviation == 0).
- populate overfull or underfull osds more aggressively by
  gradually allowing the absolute deviations converging towards
  to 0 instead of 1.

It turns out the balancer module works even better now after
applying this patch. E.g.:
```
OSD=5 MON=1 MGR=1 MDS=0 ../src/vstart.sh -x -l -b -n -d
bin/ceph osd set-require-min-compat-client luminous
bin/ceph balancer mode upmap
bin/ceph balancer on
bin/ceph osd pool create rbd 117
// wait until automatic balancing is done
bin/ceph osd pool create aaa 133
```
__before__:
```
ID CLASS WEIGHT  REWEIGHT SIZE   RAW USE DATA    OMAP META  AVAIL   %USE  VAR  PGS STATUS
 0   hdd 0.00980  1.00000 10 GiB 1.1 GiB 3.9 MiB  0 B 1 GiB 9.0 GiB 10.60 1.00 151     up
 1   hdd 0.00980  1.00000 10 GiB 1.1 GiB 3.9 MiB  0 B 1 GiB 9.0 GiB 10.60 1.00 153     up
 2   hdd 0.00980  1.00000 10 GiB 1.1 GiB 3.9 MiB  0 B 1 GiB 9.0 GiB 10.60 1.00 147     up
 3   hdd 0.00980  1.00000 10 GiB 1.1 GiB 3.9 MiB  0 B 1 GiB 9.0 GiB 10.60 1.00 149     up
 4   hdd 0.00980  1.00000 10 GiB 1.1 GiB 3.9 MiB  0 B 1 GiB 9.0 GiB 10.60 1.00 150     up
                    TOTAL 50 GiB 5.3 GiB  19 MiB  0 B 5 GiB  45 GiB 10.60
```
__after__:
```
ID CLASS WEIGHT  REWEIGHT SIZE   RAW USE DATA    OMAP META  AVAIL   %USE  VAR  PGS STATUS
 0   hdd 0.00980  1.00000 10 GiB 1.1 GiB 6.2 MiB  0 B 1 GiB 9.0 GiB 10.62 1.00 150     up
 1   hdd 0.00980  1.00000 10 GiB 1.1 GiB 6.2 MiB  0 B 1 GiB 9.0 GiB 10.62 1.00 151     up
 2   hdd 0.00980  1.00000 10 GiB 1.1 GiB 6.2 MiB  0 B 1 GiB 9.0 GiB 10.62 1.00 149     up
 3   hdd 0.00980  1.00000 10 GiB 1.1 GiB 6.2 MiB  0 B 1 GiB 9.0 GiB 10.62 1.00 150     up
 4   hdd 0.00980  1.00000 10 GiB 1.1 GiB 6.2 MiB  0 B 1 GiB 9.0 GiB 10.62 1.00 150     up
                    TOTAL 50 GiB 5.3 GiB  31 MiB  0 B 5 GiB  45 GiB 10.62
```

Fixes: http://tracker.ceph.com/issues/37940
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>