Improve `rug-ctl browse` for large-scale deployments with 500+ routers.

* Don't use RPC to list routers; this is expensive.  Instead, retrieve a list
  of limited data via the REST API.
* Bump up the number of default `nova show` workers to something reasonable
  (and make the number of threads configurable).
* Lower the frequency of SQL SELECT queries from every 1 second to every
  3 seconds to avoid overwhelming SQLite.
* Don't spawn a separate subprocess for router fetching, but instead use
  a thread; in some versions of Python, using Python's multiprocessing module
  results in odd segfaults related to our curses library.
* Sort the routers by ID so that the appliance VM details start to appear at
  the top of the list (the default view at startup) by default.
Merge pull request #191 from ryanpetrello/rug-ctl-improvements

Improve `rug-ctl browse` for large-scale deployments with 500+ routers.