MB-11517: wrongly construct failover logs

Instead of extending list elements, we need to use append function
to add additional failover logs.

Change-Id: I6ce8e8febde4eefa5f7dde1301f5b070667f19dc
Reviewed-on: http://review.couchbase.org/38753
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>