Add support for Hash Prefix

A new configuration parameter is added to /etc/swift/swift.conf
[swift-hash]
swift_hash_path_prefix = 'random unique string'

New installations are advised to set this parameter to a random secret,
which would not be disclosed ouside the organization.
The same secret needs to be used by all swift servers of the same cluster.

Existing installations should set this parameter to an empty string
(the default)

DocImpact

Fixes: Bug #1157454

Change-Id: I63b10d0b7d6dd3f74e0f10bb41b5f240fa03578a