Mark Swift blobstores as eventually consistent

This allows tests to retry correctly.  Most blobstores, including
Swift[1], offer eventual consistency, although Azure[2] and the
jclouds test blobstores offer strong consistency.

[1] https://lists.launchpad.net/openstack/msg06788.html
[2] http://blogs.msdn.com/b/windowsazure/archive/2011/11/21/windows-azure-storage-a-highly-available-cloud-storage-service-with-strong-consistency.aspx