Make inventory not mandatory

[strangeman]

We store inventory files directly in playbooks repo (and throw them to the Semaphore via Extra CLI Arguments "-i", "inventory/test/"). I created empty inventory to pass around mandatory inventory field in task templates, but for each task call, Semaphore creates empty inventory_task_id file in the Semaphore directory. Currently, it looks like that and it a bit annoying:

Can we make inventory not mandatory and not create any files, if inventory not defined?

[matejkramny]

semaphore also needs to clean up after itself!

[twhiston]

Closing in favour of #47 which should provide generalised inventory sources support, including none.