skip expensive spec tests by default

- `rake spec` skips knife integration tests
- `rake spec:all` includes them
- PRs only get `rake spec`
- master (and 10/11/12-stable) get `rake spec:all`
- rspec and rspec spec are still essentially rake spec:all
  (so omnibus-chef still tests every spec)
- also some cleanup of the rspec jobs
Merge pull request #2421 from opscode/lcg/faster-rspec

skip expensive spec tests by default