mon/OSDMonitor: make default false-positive-probability 5%

This is a more conservative default (as in, less memory consumed) for
newly created cache pools.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1331 from ceph/wip-cache-pool

mon/OSDMonitor: make default false-positive-probability 5%

Reviewed-by: Greg Farnum <greg@inktank.com>