implement :create_subfolders attribute

This pull requests creates an :create_subfolders attribute for the lwrp (defaults to true), that skips the creation and usage of the private and certs subdirectories.

This flag is useful in scenarios where you want to use the cookbook to deploy certificates to non /etc/ssl directories or different services. PostgreSQL e.g. expects the certificates as server.key and server.crt files in its data-dir.

As this flag defaults to true, it shouldn't break any existing configuration.