Make is_admin_available not call for a token

It was only doing this when configured with no tenant isolation and
no accounts file.

Change-Id: I39e461cbb7df58923b26684438a4de52ddeb8187
diff --git a/tempest/common/credentials.py b/tempest/common/credentials.py
index 3794b66..2f7fb73 100644
--- a/tempest/common/credentials.py
+++ b/tempest/common/credentials.py
@@ -58,7 +58,8 @@
             is_admin = False
     else:
         try:
-            cred_provider.get_configured_credentials('identity_admin')
+            cred_provider.get_configured_credentials('identity_admin',
+                                                     fill_in=False)
         except exceptions.InvalidConfiguration:
             is_admin = False
     return is_admin