osd/OSD: tolerate any 'set-device-class' error on OSD startup

osd/OSD: tolerate any 'set-device-class' error on OSD startup

We might use 'ceph-disk prepare --crush-device-class' to specify a class
for an OSD and then manually change its class to quite another.

This will prevent OSD from booting on next startup:

mon_cmd_maybe_osd_create fail: 'osd.2 has already bound to class 'class2', can not reset class to 'class1'; use 'ceph osd crush rm-device-cl
ass <osd>' to remove old class first': (16) Device or resource busy
2017-08-04 15:52:41.997158 7f7c55aa4d00 -1 osd.2 231 init unable to update_crush_device_class: (16) Device or resource busy

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