allow tunable superuser for pgstatsmon setup routines

[KodyKantor]

@davepacheco has a Postgres configuration where the superuser's role name isn't 'postgres.' pgstatsmon currently assumes that the Postgres superuser will always be 'postgres.' The setup routines will fail in configurations with different superusers, leaving pgstatsmon in a loop of trying to connect to Postgres as the wrong superuser.

We should add a configuration field to pgstatsmon so operators can specify which role to use to execute pgstatsmon's superuser startup routine.