Simplify creation of custom runners

At the moment //labgrid/run is the default runner that does quite a lot of things. It would be useful to extract some of its components so they can be used by custom implementations.

Components to refactor:

• Create a library for the run function.
  • Custom implementations could use wrap this run function while providing a different CLI. This should be a quick and easy way to implement a custom binary.
• Create utility function to automatically transfer runfiles along with any transferred file
• Create utility function to run a program with env vars
• Finally, run.py can be refactored so that its layout flows in a more readable way.