Rework run_shell_command

A previous patch for windows support made the simple case of passing a
string to run_shell_command complicated with quoting. Clean that up by
reverting to list, which avoids the crazy quoting we had to do.

Change-Id: Ia3b8ac6a57e7400d0aab0d5265c851e536fb4e87
Add a test for command registration.

This would be better as a direct unit test, but I can't see where
thats done; and setuptools doesn't seem super friendly to that.

There are more commands that can be tested, but this seemed like a
good place to start.

Change-Id: I433880d9b08d07de0194450bbc90a423554e347c
Merge "Add a test for command registration."
Merge "Rework run_shell_command"