Update builder files when they exist

This change makes the swift_rings.py script update builder files if they
exist. However there are some limitations:

1. Once a ring has been defined with a partpower, this cannot be changed
   without rebuilding the ring. So rule is, pick a partpower based on
   future use of you swift cluster.

2. When modifiying hosts using swift-builder you cannot change the
   region or zone. These are defined only when a new host is added. As
   this can cause major partition placement issues on existing hosts
   (will cause some major movement in the swift cluster).

   If you want to change a hosts zone or region, the best thing to do is
   set the wieght to 0 and let swift move data off the drive.. then
   remove it and add it again as a new host.

NOTE: This needs testing and debugging, as I don't have a working full
      rpc environment yet.