Fix gpexpand writable external table case

Now gpexpand uses database `postgres` to store
expand information instead of a user-specific
database.
Co-authored-by: NJialun Du <jdu@pivotal.io>

gpMgmt/test/behave/mgmt_utils/gpexpand.feature

@@ -206,7 +206,7 @@ Feature: expand the cluster by adding more segments
         Then the number of segments have been saved
         When the user runs gpexpand with the latest gpexpand_inputfile with additional parameters "--silent"
         Then verify that the cluster has 3 new segments
-        When the user runs gpexpand against database "gptest" to redistribute
+        When the user runs gpexpand to redistribute
         Then the numsegments of table "ext_test" is 4
 
     @gpexpand_icw_db_concourse