librbd: optionally wait for a flush before enabling writeback

Older guests may not send flushes properly (i.e. never), so if this is
enabled, rbd_cache=true is safe for them transparently.

Disable by default, since it will unnecessarily slow down newer guest
boot, and prevent writeback caching for things that don't need to send
flushes, like the command line tool.

Refs: #3817
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
PendingReleaseNotes: fix typo

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>