Previously the recon middleware was doing a basic scan for object
rings that exist at init time. In situations where an object-server
was started without an object ring present, but received one shortly
after, recon still would not report it in the /recon/ringmd5 response.
This persists even when object-server gleefully chugs along after
picking up the ring, and recon's behavior would only be corrected by
an object-server reload/restart.
This change brings the middleware a bit more up to date to use the
common POLICIES instance to determine what policies were already loaded
based on configuration, and derives the path for each ring.
This effectively makes the config the source of truth for what rings
*should* be present, rather than what's present at startup. Since we
already dynamically check in ReconMiddleware.get_ring_md5 whether each
of the predetermined ring files exist, recon now correctly reports a
previously-missing ring whenever it falls into place.
Change-Id: Ia079418e54ffac5e01ef6a15511f5069b7fe83ea
460a7e4b64