Refactored lists of nodes to contact for requests

Extensive refactor here to consolidate what nodes are contacted for
any request. This consolidation means reads will contact the same set
of nodes that writes would, giving a very good chance that
read-your-write behavior will succeed. This also means that writes
will not necessarily try all nodes in the cluster as it would
previously, which really wasn't desirable anyway. (If you really want
that, you can set request_node_count to a really big number, but
understand that also means reads will contact every node looking for
something that might not exist.)

* Added a request_node_count proxy-server conf value that allows
  control of how many nodes are contacted for a normal request.

In proxy.controllers.base.Controller:

* Got rid of error_increment since it was only used in one spot by
  another method and just served to confuse.

* Made error_occurred also log the device name.

* Made error_limit require an error message and also documented a bit
  better.

* Changed iter_nodes to just take a ring and a partition and yield
  all the nodes itself so it could control the number of nodes used
  in a given request. Also happens to consolidate where sort_nodes is
  called.

* Updated account_info and container_info to use all nodes from
  iter_nodes and to call error_occurred appropriately.

* Updated GETorHEAD_base to not track attempts on its own and just
  stop when iter_nodes tells it to stop. Also, it doesn't take the
  nodes to contact anymore; instead it takes the ring and gets the
  nodes from iter_nodes itself.

Elsewhere:

* Ring now has a get_part method.

* Made changes to reflect all of the above.

Change-Id: I37f76c99286b6456311abf25167cd0485bfcafac
Merge "Refactored lists of nodes to contact for requests"