implemented 'inventory_file' variable. Closes GH-3789.

We use the hosts file like our "environment" like `hosts/production` has all production hosts and  `hosts/testing` has all testing hosts, but same groups. 

So testing becomes very natural:

    ansible-playbook -i hosts/testing playbook.yml

if testing is ok, apply to production 

    ansible-playbook -i hosts/production playbook.yml

But sometimes it makes sense to know in which "environment" (from which inventory_file) you apply the task.

e.g. this would be very nice:

    action: foobar...
    when: {{ inventory_file }} is "hosts/production"

Cheers 
René