Implement GPG Keys cli positive create stubs

Also:
* fix a bug in cli.GPGKey list method
* improve make_gpg_key factory to accept a local file path and upload it

The tests as passing when the `redminebug` decorator is commented. Leaved there to track those opened bugs. Follow the output from the running tests without the decorator:

```
$ nosetests -c robottelo.properties tests/cli/test_gpgkey.py -m test_positive_create
INFO ssh:61: Paramiko instance prepared (and would be reused): 0x10ed80210
@feature: GPG Keys ... ok
@feature: GPG Keys ... ok
@feature: GPG Keys ... ok
@feature: GPG Keys ... ok
@feature: GPG Keys ... ok
@feature: GPG Keys ... ok
@feature: GPG Keys ... ok
@feature: GPG Keys ... ok
@feature: GPG Keys ... ok
@feature: GPG Keys ... ok
@feature: GPG Keys ... ok
@feature: GPG Keys ... ok

----------------------------------------------------------------------
XML: foreman-results.xml
----------------------------------------------------------------------
Ran 12 tests in 183.587s

OK
```

This PR is related to #422