New bot which executes commands with local tools in a temp directory.
Bots have been moved to a new folder to act as plugins for BuildGrid.

Also some small fixes to returned work from bots to execution.

Dummy command updated.
New CAS demo added.

The 3.7.x releases seem to suffer from a regression in tests collection
that affects our linting process. It may have been fixed upstream
already [1]. Freezing requirement to latest 3.6.x for now.

[1] https://github.com/pytest-dev/pytest/pull/3802