Manifest mock, and fixed tests.foreman.cli.test_subscription

Usage:

You need to have ```manifest.key_url```, ```manifest.fake_url``` and ```manifest.cert_url``` set in your robottelo.properties

My config can be found on:
http://pastebin.test.redhat.com/230135 

With updated config try

```nosetests -c robottelo.properties tests.foreman.cli.test_subscription```

and it should run :)
Can this docstring be rewritten to better explain what's going on? Based on this docstring, I don't know what's being done or what is returned. Of course, I could read the source - but the point of a docstring is that I don't have to do so.