travis: use bionic as default for Linux (#16521)

Modifications:

- [x] Travis now uses `bionic` as a default (`xenial` used for Python 3.5, `trusty` for Python 2.6)
- [x] Shell unit tests have been factored into their own run
- [x] `kcov` is built only for tests that upload coverage results

Overall with this we shave 3-4 mins. on each run and add an additional run of about 3 min. For some reason `kcov` 38 fails forwarding output when used with Python unit tests, so I used v34 for that and v38 (latest) for shell testing. Previously we were using v25.