[rotang] Add a generator to generate shifts per member TZ.

This will build shifts with on member from each timezone.

Eg.
  If a rotation have members from UTC and US/Pacific timezones this
  generator will build shifts with one member from the UTC TZ and one
  from the US/Pacific tz.

Change-Id: I89d5a0278ef60ffba43a234c462c5117893cffa9
Reviewed-on: https://chromium-review.googlesource.com/c/1460376
Reviewed-by: Scott Lewis-Kelly <slewiskelly@google.com>
Commit-Queue: Ola Karlsson <olakar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#20536}