MB-7965: Setting flushall_enabled to true by default

Change-Id: I0b6bd61fc2a1e07971b26bf6ec562cdebc5097fb
Reviewed-on: http://review.couchbase.org/43970
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>