'Look for environment variables describing an existing stack but skip
the test unless *all* the required variables are supplied.'

def _stack_from_environment(self):

    found = {}
    for variable_name in STACK_VARIABLES.keys():
        value = os.environ.get(variable_name.upper(), None)
        if (value is not None):
            found[variable_name] = value
            setattr(self, variable_name, value)
    if found:
        missing_keys = (set(STACK_VARIABLES.keys()) - set(found.keys()))
        if missing_keys:
            raise Exception('Missing    Keys', missing_keys)
        else:
            return True
    else:
        return False
