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
1 file changed