Rename the 'path' argument of the MasterShellCommand 'workdir'

Few considerations:

- All other steps take a workdir
- The path argument is not mentioned in the doc
  - Even worst, the doc has an example: command="cd dir; do stuff"
- As the test show, the one that wrote it believed it would become the PATH.
These are definitely wrong - http://twistedmatrix.com/documents/current/api/twisted.internet.interfaces.IReactorProcess.spawnProcess.html says `path` is the working directory in which to run the command -- not a list.