Clean up get_cmd_path/make_cmd_path/make_cmd_dirs mess

Clean up the Plugin helper methods for handling command output
paths. This better matches the proposal in Issue #181 and
simplifies code for plugins that need to generate their own
paths under sos_commands/<plugin>/.

- Rename get_cmd_path() to get_cmd_output_path()
- Add an optional 'name' parameter to specify a subdirectory
  name in the plugin's sos_commands/<plugin>/ directory.
- Default to creating directories if they do not exist

Finally fix up all existing users of get_cmd_path() to use the
new interface and remove handrolled uses of os.path.join(...) for
generating these paths.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>