check is_admin_available for force_tenant_isolation in test.py

If force_tenant_isolation is set to True, admin credential is
needed, and this check should be put in skip_checks in test.py,
while should not be done by individual testcases.

Change-Id: I9506120a088cde270b9c787bab26bf6e4c4a23d3
diff --git a/tempest/test.py b/tempest/test.py
index f390ae4..a4cc2cc 100644
--- a/tempest/test.py
+++ b/tempest/test.py
@@ -220,10 +220,13 @@
         resource_setup or at test level.
         """
         identity_version = cls.get_identity_version()
-        if 'admin' in cls.credentials and not credentials.is_admin_available(
-                identity_version=identity_version):
-            msg = "Missing Identity Admin API credentials in configuration."
-            raise cls.skipException(msg)
+        # setting force_tenant_isolation to True also needs admin credentials.
+        if ('admin' in cls.credentials or
+                getattr(cls, 'force_tenant_isolation', False)):
+            if not credentials.is_admin_available(
+                    identity_version=identity_version):
+                raise cls.skipException(
+                    "Missing Identity Admin API credentials in configuration.")
         if 'alt' in cls.credentials and not credentials.is_alt_available(
                 identity_version=identity_version):
             msg = "Missing a 2nd set of API credentials in configuration."