Merge pull request #12798 from wjwithagen/wip-wjw-krbd-optional

rbd: enable rbd on FreeBSD (without KRBD)

Reviewed-by: Kefu Chai <kchai@redhat.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>

diff --cc CMakeLists.txt
Simple merge

diff --cc src/include/config-h.in.cmake
Simple merge

diff --cc src/test/librbd/operation/test_mock_SnapshotCreateRequest.cc
Simple merge