merge changes from master 

Merge pull request tox-dev#574 from Avira/master fix up tox-dev#521: add missing pass of arg to underlying function.

Show default text for missing descriptions in `-[l|a]v` 

release 2.6.0 

release 2.5.0 

2.4.1 release 

2.4.0 release 

fix issue294: re-allow cross-section substitution for setenv setting. 

deprecate indexserver options