Allow to soft-delete keys during config upgrade.

Change-Id: If36cc134de89d97ec2294e5759b5338d71a99cbc
Reviewed-on: http://review.couchbase.org/35289
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>