mon/OSDMonitor: optional show-shadow for "crush tree" command

mon/OSDMonitor: optional show-shadow for "crush tree" command

 ./bin/ceph osd crush tree --show-shadow
ID WEIGHT  TYPE NAME
-4 3.00000 root default~ssd
-3 3.00000     host gitbuilder-ceph-rpm-centos7-amd64-basic~ssd
 0 1.00000         osd.0
 1 1.00000         osd.1
 2 1.00000         osd.2
-1 3.00000 root default
-2 3.00000     host gitbuilder-ceph-rpm-centos7-amd64-basic
 0 1.00000         osd.0
 1 1.00000         osd.1
 2 1.00000         osd.2

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