rbd should use write-back when caching is enabled

librbd now defaults to write-through until the first flush
is received.  For rbd, force the use of write-through.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
Merge pull request #2433 from ceph/wip-rbd-force-write-back

rbd should use write-back when caching is enabled

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>