Environment variables required across unrelated environments

[AndreLouisCaron]

Hi there,

I have an issue where if one environment uses an {env:X} subsitution (without default), then that environment variable is required in all environments regardless of whether they are necessary or not.

Given this example tox.ini file:

[tox]
skipsdist = true

[testenv:standard-greeting]
commands =
  python -c 'print("Hello, world!")'

[testenv:custom-greeting]
passenv =
  NAME
commands =
  python -c 'print("Hello, {env:NAME}!"'

Then custom-greeting environment cannot be used unless NAME is present in the calling environment, even if that environment doesn't use the {env:NAME}.

$ tox -e standard-greeting
tox.ConfigError: ConfigError: substitution env:'NAME': unknown environment variable 'NAME'  or recursive definition.

I got this with tox 2.7.0 with both Python 3.5 and 2.7 on Windows.