Fix _get_creds for alt user

Related-Prod: PRODX-26490
Change-Id: Iacbf20f3b0422370b23c22c9f476c7da48fb5520
diff --git a/tempest/lib/common/preprov_creds.py b/tempest/lib/common/preprov_creds.py
index 508618a..6c23a08 100644
--- a/tempest/lib/common/preprov_creds.py
+++ b/tempest/lib/common/preprov_creds.py
@@ -372,7 +372,8 @@
     def get_alt_creds(self):
         if self._creds.get('alt'):
             return self._creds.get('alt')
-        net_creds = self._get_creds()
+        # NOTE(pas-ha) use the same call as get_project_member_creds
+        net_creds = self._get_creds(['member'], scope='project')
         self._creds['alt'] = net_creds
         return net_creds