Add support for 'config_entries' in postgresql::server

This adds a parameter 'config_entries' to postgresql::server.
It allows the user to add an arbitrary number of configuration parameters to postgresql.conf directly with the postgresql::server class.

This is nice to have, for example, when using Hiera or an ENC  where you can add all your configuration entries without resorting to  a wrapper class that has to declare them individually.

The old methods of adding entries are left untouched, and by default the value of the parmeter is an empty hash (no entries).