GUAC-586: Associate unique identifier with each AuthenticationProvider.

extensions/guacamole-auth-jdbc/modules/guacamole-auth-jdbc-postgresql/src/main/java/org/glyptodon/guacamole/auth/postgresql/PostgreSQLAuthenticationProvider.java

@@ -192,6 +192,11 @@ public class PostgreSQLAuthenticationProvider implements AuthenticationProvider
 
     }
 
+    @Override
+    public String getIdentifier() {
+        return "postgresql";
+    }
+
     @Override
     public AuthenticatedUser authenticateUser(Credentials credentials)
             throws GuacamoleException {