Fix bug where rose-stem cannot ascertain name of a suite when cwd is not a working copy

If the current working directory where "rose stem" is invoked is not a working copy, the command will currently fail as it attempts to work out a sensible suite name from the base of the current working copy. 

This generally only occurs when running rose stem with both the --source and --config options, but that is occasionally necessary. This change alters the command so if it is not possible to ascertain a sensible basename of the working copy, it takes the directory specified by --config and uses the basename of that (replacing "." with the actual name), or failing that the basename of the directory in which the user ran the command. 

Can you get this from `os.path.abspath`?