By default listings are rendered with a label of:
"Listing of /v1/account/container/path"
This change adds a new custom metadata key/value pair
"X-Container-Meta-Web-Listings-Label: <label>" that
when set, will cause the following:
"Listing of <label>/path" to be rendered instead.
Change-Id: I47c5d21834f47e566b380920196926882ff83abf