Project creator: use absolute path for json config files

The original config file path depends on the current directory.

When the current directory is not `$CC_ROOT/tools/project_creator/`, only template files will be copied, leaving further operations undone (filenames untouched etc)...

    Traceback (most recent call last):
      File "E:\repo\cocos2d-x\tools\project_creator\create_project.py", line 175, in <module>
        createPlatformProjects()
      File "E:\repo\cocos2d-x\tools\project_creator\create_project.py", line 160, in createPlatformProjects
        processPlatformProjects(context, platform)
      File "E:\repo\cocos2d-x\tools\project_creator\create_project.py", line 99, in processPlatformProjects
        f = open("%s.json"%platform)
    IOError: [Errno 2] No such file or directory: 'ios.json'

I changed the relative path to absolute path when opening the JSON config file.