When issuing `delete` commands that would require three or more
individual deletes, check whether the cluster supports bulk deletes
and use that if it's available.
Additionally, a new option is added to the `delete` command:
* --prefix <prefix>
Delete all objects that start with <prefix>. This is similar to the
--prefix option for the `list` command.
Example:
$ swift delete c --prefix obj_prefix/
...will delete from container "c" all objects whose name begins with
"obj_prefix/", such as "obj_prefix/foo" and "obj_prefix/bar".
Change-Id: I6b9504848d6ef562cf4f570bbcd17db4e3da8264
7a1e192803