Add tests with big backend

Basic tests can be executed with 15 machines Master/slave backend. Tests have label BIG_REPL_BACKEND Default template modified to support big backend. Tests temporaraly labeled as UNSTABLE to prevent their execution nightly

maxscale-system-test/bug587.cpp

@@ -72,6 +72,7 @@ using namespace std;
 int main(int argc, char* argv[])
 {
     TestConnections* Test = new TestConnections(argc, argv);
+    Test->repl->limit_nodes(4);
     Test->set_timeout(10);
     Test->repl->connect();
     Test->maxscales->connect_maxscale(0);