full_stack-insert-scan-test: allow customization of number of tservers

Also snuck in some gscoped_ptr -> unique_ptr changes.

Change-Id: Ibf903b058e8ff4fd87c5897ab261c9bc6f87fc06
Reviewed-on: http://gerrit.cloudera.org:8080/15094
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <aserbin@cloudera.com>
1 file changed