Rename groupmaster to groupleader and related variables. (#1875)

Fix the naming conventions. 
Master should be reserved to identify a bots Master. 
groupleaders are not necessarily group masters and it should be clear
what the bot is looking for. (In most solo cases leader=master)

src/strategy/values/GroupLeaderValue.cpp

@@ -0,0 +1,10 @@
+/*
+ * Copyright (C) 2016+ AzerothCore <www.azerothcore.org>, released under GNU AGPL v3 license, you may redistribute it
+ * and/or modify it under version 3 of the License, or (at your option), any later version.
+ */
+
+#include "GroupLeaderValue.h"
+
+#include "Playerbots.h"
+
+Unit* GroupLeaderValue::Calculate() { return botAI->GetGroupLeader(); }