Enabled nested configuration files

Allowing hierarchical configuration files will make the management of MaxScale's configuration a lot cleaner. This is in line with the MariaDB server which has the /etc/my.cnf.d/ folder. MaxScale should have a /etc/maxscale.cnf.d/ folder which has all the relevant configuration files.

When provided with a configuration file xyz.cnf, MaxScale should automatically and recursively read all cnf-files from the directory xyz.cnf.d and all its subdirectories.