Move path to ssl_server_public to pe_role

Since it is a variable used by multiple subclasses, make it a variable
in the pe_role class. Not a parameter - this is not something we want
exposed to user tuning - but a variable.
Add middleware hub role

This serves more as an example than anything else of how to define
connections and collect exported connectors. It can be used as-is in
simple, single-datacenter deployments.