Work around failed metadata lookups with Rackspace

Previously BlobStore.list and thus BlobStore.clearContainer serialized
on synchronous and failed CDN lookups.  This effectively prevents
clearing any large container.  We cache a negative entry to work
around this.  This commit is similar to commit 3c7b965 to
hpcloud-objectstorage.
Merge pull request #1194 from maginatics/rax-list-container

Work around failed metadata lookups with Rackspace