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`
Actually, won't rspec look for `.rspec` in the project root by default?  https://github.com/rspec/rspec-core/blob/master/lib/rspec/core/configuration_options.rb#L118-L120