crush: get "ceph osd crush class create/rm" back

crush: get "ceph osd crush class create/rm" back

This reverts a27fd9d25cb2819e25cc48b790c40afac0250464 and
b863883ca783487401fde4f4480ed1d9b093363e.

Quote form Sébastien Han:
> IIRC at some point, we were able to create a device class from the CLI.
Now it seems that the device class gets created when at least one OSD
of a particular class starts.
In ceph-ansible, we create pools after the initial monitors are up and
we want to assign a device crush class on some of them.
That's not possible at the moment since there no device class available yet.
Also, someone might want to create its own device class.
Something as crazy as running Filestore with a tmpfs osd store and
might want to isolate them.
I know it's a very limited use case, but still, it could be desired.

See also https://www.spinics.net/lists/ceph-devel/msg41152.html

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