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`
it would be nice if we could default `rspec_opts` for all tasks, but I don't see anything in the rspec documentation that allows that.