Support custom metadata in object_store

[Brian Curtin]

We should be able to set and get user-specified custom metadata on object_store resources. Currently we can only work with documented properties, but we should be able to work with arbitrary names and properly handle the insertion and removal of, e.g., X-Container-Meta-{name}, while getting and setting.

A user should be able to set and get something like "my-favorite-band" and not have to prepend anything on the set or remove anything on the get.

[Terry Howe (thowe-g)]

Agreed.