Added discoverable capabilities.

Swift can now optionally be configured to allow requests to '/info',
providing information about the swift cluster.  Additionally a HMAC
signed requests to
'/info?swiftinfo_sig=<sign>&swiftinfo_expires=<expires>' can be
configured allowing privileged access to more sensitive information
not meant to be public.

DocImpact
Change-Id: I2379360fbfe3d9e9e8b25f1dc34517d199574495
Implements: blueprint capabilities
Closes-Bug: #1245694
Merge "Added discoverable capabilities."