Implement GPG Keys cli negative create stubs

Also remove stubs related to UI (copy/paste scenarios)

Remove the stubs test_negative_create_2 and test_negative_create_5 and renamed other stubs:

* test_negative_create_3 -> test_negative_create_2
* test_negative_create_4 -> test_negative_create_3

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_negative_create   
INFO ssh:61: Paramiko instance prepared (and would be reused): 0x1952d90
@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
@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 19 tests in 257.011s

OK
```

This PR is related to #435