Replace the usage of 'manager' with 'os_primary'

Alias 'manager' has been moved to 'os_primary' in version Pike,
and it will be removed in version Queens. This patch is to replace
the usage of 'manager' with 'os_primary'.

Change-Id: I5301287b3c6265a6f235d63b3665199065bde0d4
diff --git a/tempest/api/identity/admin/v3/test_tokens.py b/tempest/api/identity/admin/v3/test_tokens.py
index 8c5e63b..1acc67d 100644
--- a/tempest/api/identity/admin/v3/test_tokens.py
+++ b/tempest/api/identity/admin/v3/test_tokens.py
@@ -145,7 +145,7 @@
 
     @decorators.idempotent_id('08ed85ce-2ba8-4864-b442-bcc61f16ae89')
     def test_get_available_project_scopes(self):
-        manager_project_id = self.manager.credentials.project_id
+        manager_project_id = self.os_primary.credentials.project_id
         admin_user_id = self.os_admin.credentials.user_id
         admin_role_id = self.get_role_by_name(CONF.identity.admin_role)['id']