extend selector with EXISTS, new tests/some old tests use table-driven style

Based on discussion in https://github.com/GoogleCloudPlatform/kubernetes/pull/715. Some of the old tests were not converted to table-driven format since the code on which they are based will probably be removed in the near term. Future work includes: changing uses of old code in other parts of the code base to use newer version of selector, removing old selector code, removing tests of old selector code, and adding number ranges to selector values.
@bgrant0607 I have included comments on the latest commit from today (https://github.com/GoogleCloudPlatform/kubernetes/commit/622aae6d6ea660074c3e64394c18de928edbd8f6) that reflect the exact match distinction. To create the type for the new set-based matching and use that in v1beta3 I think we should do further work on completing set-based matching in subsequent PR(s) that would also include renaming and/or deprecating/removing the old matching.