Merge pull request #12536 from ilc/cls_rgw_minor_refactor

cls/rgw: Clean up the "magic string" usage in the cls layer for RGW.

Reviewed-by: Casey Bodley <cbodley@redhat.com>
Reviewed-by: Adam C. Emerson <aemerson@redhat.com>

diff --cc src/cls/rgw/cls_rgw.cc
Simple merge

diff --cc src/cls/rgw/cls_rgw_client.cc
Simple merge

diff --cc src/rgw/rgw_rados.cc
Simple merge