[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/model/ModelBase.cpp

@@ -0,0 +1,66 @@
+/**
+* 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 "ModelBase.h"
+
+namespace org {
+namespace openapitools {
+namespace server {
+namespace model {
+
+ModelBase::ModelBase()
+{
+}
+ModelBase::~ModelBase()
+{
+}
+
+std::string ModelBase::toJson( const std::string& value )
+{
+    return value;
+}
+
+std::string ModelBase::toJson( const std::time_t& value )
+{
+    char buf[sizeof "2011-10-08T07:07:09Z"];
+    strftime(buf, sizeof buf, "%FT%TZ", gmtime(&value));
+    return buf;
+}
+
+int32_t ModelBase::toJson( int32_t value )
+{
+    return value;
+}
+
+int64_t ModelBase::toJson( int64_t value )
+{
+    return value;
+}
+
+double ModelBase::toJson( double value )
+{
+    return value;
+}
+
+bool ModelBase::toJson( bool value )
+{
+    return value;
+}
+
+nlohmann::json ModelBase::toJson( std::shared_ptr<ModelBase> content )
+{
+    return content.get() ? content->toJson() : nlohmann::json();
+}
+
+}
+}
+}
+}