Update ambari admin credentials for scaling

If the cluster is created with alternate credentials for the Ambari
admin use those alternate credentials (rather than the default
'admin/admin' combo) for the scaling operations.

Fixes:  bug #1222542

Change-Id: Ib06906e3f37ec7ee71a04dc1f23cd3698fc33245
(cherry picked from commit d3edcdb996a0188edba44691cb98a0d74a42de4c)