Fix Postgres connect, incorrect port value (#5618)

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##### Summary
Fix postgres connection, port value was incorrect, it was taking host value instead of port.

##### Component Name
[collectors/python.d.plugin/postgres](https://github.com/netdata/netdata/tree/master/collectors/python.d.plugin/postgres)

##### Additional Information

collectors/python.d.plugin/postgres/postgres.chart.py

@@ -815,7 +815,7 @@ class Service(SimpleService):
 
         params = {
             'host': conf.get('host'),
-            'port': conf.get('host', DEFAULT_PORT),
+            'port': conf.get('port', DEFAULT_PORT),
             'database': conf.get('database'),
             'user': conf.get('user', DEFAULT_USER),
             'password': conf.get('password'),