The middleware now allows the usage of signatures with a prefix-based
scope. A prefix-based signature grants access to all objects which share
the same prefix. This avoids the creation of a large amount of signatures,
when a whole container or pseudofolder is shared.
Please see spec: https://review.openstack.org/#/c/199607/
Change-Id: I03b68eb74dae6196b5e63e711ef642ff7d2cfdc9