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 :)
When dealing with file paths, is better to use `os.path` module.