[C++ server] Adjust the names (script, sample folder, generator) to lang option (#250)

* Renamce C++ server scripts

* Change output folder

* Rename sample folder: pistache-server -> cpp-pistache

* Rename sample folder: restbed -> cpp-restbed

samples/server/petstore/cpp-pistache/PetApiMainServer.cpp

@@ -0,0 +1,29 @@
+/**
+* OpenAPI Petstore
+* This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+*
+* OpenAPI spec version: 1.0.0
+* 
+*
+* NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+* https://openapi-generator.tech
+* Do not edit the class manually.
+*/
+
+#include "pistache/endpoint.h"
+#include "pistache/http.h"
+#include "pistache/router.h"
+#include "PetApiImpl.h"
+
+using namespace org::openapitools::server::api;
+
+int main() {
+    Pistache::Address addr(Pistache::Ipv4::any(), Pistache::Port(8080));
+
+    PetApiImpl server(addr);
+    server.init(2);
+    server.start();
+
+    server.shutdown();
+}
+