Adds swift_hash_path_prefix option in swift conf

The [swift-hash] section of the object storage installation guide
contains swift_hash_path_suffix option, but misses swift_hash_path_prefix
option. This patch adds it.

Change-Id: Id5e13db91c94ac794830f87acf6ecb970cef7556
Closes-Bug: #1332111
(cherry picked from commit 9d070343dc15fccd03fab200ec0920e5eaeb6a3d)