Set default value for tox tasks

This commit sets a default value '-' for the tox tasks. Users can run
the tox tasks all, all-plugin and all-sitepackages without specifying
regexes. Without this patch, users face such an error when users run
the tox tasks without specifying regexes.

Change-Id: Ia523895e8f66f25e9a4103cb9be1f1c3fc3f1b62
1 file changed