Instead of using a Tox axis, expand this pattern to retrieve the Tox environment to execute.
Typically used with User-defined Axis, for instance an INTERPRETER="py27 py32" one and a DATABASE="mysql postgres" one.
In this case setting TOXENV pattern to ${INTERPRETER}-${DATABASE} will execute the following Tox environments: py27-mysql, py27-postgres, py32-mysql and py32-postgres.