crush: "osd crush class rename" support

crush: "osd crush class rename" support

In 076a6abd80cc90ebcb901f908f880ef030721b2a I killed the 'class rename' command
and thought it was totally useless but I was wrong.

Consider the following user case:
(1) randomly choose some OSDs(e.g., from different hosts) and try to make them for private use only,
    say, by grouping them into 'pool1'
(2) ceph osd crush set-device-class pool1 'OSDs from (1)'
(3) ceph osd crush rule create-replicated rule_for_pool1 default host pool1
(4) ceph osd pool rename pool1 pool2
(5) ceph osd crush class rename pool1 pool2

From the above user case, we need to safely change a pool name without worrying
any risk of data migration. That is why the 'osd crush class rename' command
is still needed here.

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