Add a region tier to Swift's ring.

The region is one level above the zone; it is intended to represent a
chunk of machines that is distant from others with respect to
bandwidth and latency.

Old rings will default to having all their devices in region 1. Since
everything is in the same region by default, the ring builder will
simply distribute across zones as it did before, so your partition
assignment won't move because of this change. If you start adding
devices in other regions, of course, the assignment will change to
take that into account.

swift-ring-builder still accepts the same syntax as before, but will
default added devices to region 1 if no region is specified.

Examples:

$ swift-ring-builder foo.builder add r2z1-1.2.3.4:555/sda

$ swift-ring-builder foo.builder add r1z3-1.2.3.4:555/sda

$ swift-ring-builder foo.builder add z3-1.2.3.4:555/sda

Also, some updates to ring-overview doc.

Change-Id: Ifefbb839cdcf033e6c9201fadca95224c7303a29