plugins: disable support for config dir

When removing and installing again the plugin all configuration files will be removed in `config/pluginname` dir.
This is bad as users may have set and added specific configuration files.

During an install, if we detect already existing files in `config/pluginname` directory, we simply copy the new file to the same dir but we append a `.new` at the end.

Related to  #5064.

(cherry picked from commit 5da028f)
(cherry picked from commit 4cb1f95)