Add Pytest based test executor

[mtreinish]

In the python testing world the stdlib untittest testing library and runner are not the defacto standard for running tests, most of the python world uses pytest. In recent years the testing cabal stdlib unittest extension stack that we've built stestr around hasn't been super actively maintained and new features in stdlib unittest aren't supported at all. To enable broader adoption of stestr and also enable using more advanced testing features we should look into adding a mode of operation that leverages pytest as a test runner instead of our unittest based runner. We should still continue to maintain, support, and default to the current test execution model as that's what our users expect. But having a way to configure stestr to use pytest for execution instead would be a great feature. This would enable users to get all the advanced parallelism and scheduling features that stestr provides and the more sophisticated test library features that pytest offers.

The only requirement in practice to make this happen in practice is to have pytest emit a real time subunit stream as it executes tests. If we can teach pytest how to do this then in theory it's just a drop in replacement inside of stestr to call pytest instead of python -m stestr.subunit_runner.run (which is part of stestr's heritage being forked from testr that supports working like a test runner runner).

The open questions for me are on how we package this (do we make pytest a hard requirement) and include a plugin to enable subunit output natively, or do we leave the subunit plugin as an external library and make pytest an optional dependency?

Additional context

@jogo started an implementation of this last year which relied on a fork of an existing pytest-subunit plugin for adding subunit support to pytest. We can take over where he left off and figure out how to best integrate this into stestr.

https://github.com/jogo/pytest-subunit
https://github.com/jogo/stestr/tree/pytest