Skip image copy test when only 1 store is available

On environments where there is only one backend in enabled_backends
glance will refuse to copy image as when it is created it is uploaded
to default store.

Related-Prod: https://mirantis.jira.com/browse/PRODX-28221
Change-Id: I1c1cb52225cab65fed1ae246c83b55284a403332
diff --git a/tempest/api/image/v2/admin/test_images.py b/tempest/api/image/v2/admin/test_images.py
index 257ddf5..51b8fbe 100644
--- a/tempest/api/image/v2/admin/test_images.py
+++ b/tempest/api/image/v2/admin/test_images.py
@@ -88,7 +88,7 @@
         # NOTE(gmann): Skip if copy-image import method and multistore
         # are not available.
         if ('copy-image' not in available_import_methods or
-            not available_stores):
+            len(available_stores) < 2):
             raise self.skipException('Either copy-image import method or '
                                      'multistore is not available')
         uuid = data_utils.rand_uuid()