With `-a`: do not show additional environments header if there are none

[obestwalter]

Small nitpick and easy to fix: when I say tox -a I don't want to see e.g.:

default environments:
flake8 -> static analysis with flake8
py27   -> run tests with Python2.7

additional environments:

but only:

default environments:
flake8 -> static analysis with flake8
py27   -> run tests with Python2.7

... if no additional environments are defined.