Ignore ssl errors when using PhantomJS.

PhantomJS allows you to run all your UI tests in headless mode. I
finally figured out how to fix the issue where PhantomJS was not raising
an issue when connecting to a server via https. Ran a quick test to make
sure things worked.

```bash
$ nosetests -c robottelo.properties tests/ui/test_login.py
@Feature: Login - Negative ... ok
@Feature: Login - Negative ... ok
@Feature: Login - Positive ... ok

----------------------------------------------------------------------
XML: foreman-results.xml
----------------------------------------------------------------------
Ran 3 tests in 11.706s

OK
```

To use it, make sure to update your ``robottelo.properties`` as follows:

```INI
driver=phantomjs
```