cmake: pass tox envs to run_tox.sh explicitly

cmake: pass tox envs to run_tox.sh explicitly

* add two optional keyword parameters to `add_tox_test()`
* use the default `TOX_PATH` parameter instead passing it explicitly,
  as it is always `${CMAKE_CURRENT_SOURCE_DIR}`
* drop the code to guess the tox envs in `run_tox.sh`, and always pass
  them explicitly in CMake script using the `--tox-envs` argument.

Signed-off-by: Kefu Chai <kchai@redhat.com>