PCBC-209 - Make the default val for persistent instances tunable

Change-Id: If7258ec01cac2a0405b94674427f2842132ad387
Reviewed-on: http://review.couchbase.org/25051
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>