CBLT-316:  add kill_child_processes() to run on exit, and before test

Change-Id: I6841fd7923cc198ef3b742695e7029a531bde571
Reviewed-on: http://review.couchbase.org/38048
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>