Enable tempest run -l without credential setting

This commit moves the admin credential code to the initialization phase
of Manager class. This change enables to run `tempest run -l` without
credential settings. We shouldn't require like that settings to just
list tests.

Change-Id: Ib202880e7039113a58dc1596de54be4167c5307d
diff --git a/ironic_tempest_plugin/services/introspection_client.py b/ironic_tempest_plugin/services/introspection_client.py
index 3b1a75b..f82aed6 100644
--- a/ironic_tempest_plugin/services/introspection_client.py
+++ b/ironic_tempest_plugin/services/introspection_client.py
@@ -17,13 +17,14 @@
 
 
 CONF = config.CONF
-ADMIN_CREDS = common_creds.get_configured_admin_credentials()
 
 
 class Manager(clients.Manager):
     def __init__(self,
-                 credentials=ADMIN_CREDS,
+                 credentials=None,
                  api_microversions=None):
+        if not credentials:
+            credentials = common_creds.get_configured_admin_credentials()
         super(Manager, self).__init__(credentials)
         self.introspection_client = BaremetalIntrospectionClient(
             self.auth_provider,