Refactor configuration-related code into a new 'spreads.config' module
Heavy refactoring of plugin-related code

- Get rid of stevedore by using plain pkg_resources
- Simplify spreads.plugin API to five functions, 'get_driver',
  'get_plugins', 'get_available_drivers', 'get_available_plugins',
  'get_devices'
- Greatly simplified driver/plugin mocks in testsuite
Fix setting of core options from arguments
Speed up tests by decreasing sleep lenths and number of test captures
Always write logfile
Fix configuration type in workflow
tests: Stub out Configuration.read()
Fix some rebasing pains
web: Improve performance of index and event endpoints