mon: enable leveldb cache by default

256 is not as large as the upstream 512 MB, but will help signficiantly and
be less disruptive for existing cuttlefish clusters.

Sort-of backport of e93730b7ffa48b53c8da2f439a60cb6805facf5a.

Signed-off-by: Sage Weil <sage@inktank.com>