Convert Rammbock runner scripts from shell to python

[jkohvakk]

atest and utest runner scripts should be converted from shell scripts to python scripts. It shall be done so that changing python version in run is trivial.

[pekkaklarck]

Added simple acceptance test runner in d7c1cbf and enhanced it in a7ba130 (that I forgot to link to this issue). The new script is in atest/run.py and ought to support all the same features as the old one plus some additional goodies. You can see help with --help.

How does the script look like @jkohvakk? If it's good, we can delete the old execute_regression_tests.sh.

Is running tests explained somewhere? If yes, docs need to be updated too. If not, perhaps we could add atest/README.rst or something similar.

Plan to add unit test runner to utest/run.py. No time to do it right now.

[pekkaklarck]

Above commits added utest runner and enhanced atest runner. I consider them ready but would like a second opinion before removing old runner scripts.

The main remaining task is changing acceptance execution so that it runs unit tests using the same interpreter as used for running acceptance tests. Currently unit tests are run in atest/misc.robot, but it would be a lot easier if atest/run.py executed them before starting acceptance test execution. Would that be fine @jkohvakk? If we do that, what should we do with the current PEP-8 tests also in atest/misc.robot? I'd like to integrate them with unit tests somehow.