os/bluestore: allow user to unset pool compression_mode[algorithm] properly

os/bluestore: allow user to unset pool compression_mode[algorithm] properly

A string-typed pool option requires user pass in a empty string to do a valid cancellation,
but the "osd pool set " command won't allow it. E.g.:
./bin/ceph osd pool set rbd compression_mode
Invalid command:  missing required parameter val(<string>)

Since we already use the "unset" keyword to cancel the csum_type setting,
we could simply extends the above mechanism for compression_mode
and compression_algorithm too.

E.g.:
./bin/ceph osd pool set rbd compression_algorithm zlib
set pool 0 compression_algorithm to zlib

 ./bin/ceph osd pool set rbd compression_algorithm unset
unset pool 0 compression_algorithm

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