maint: Slightly refactor facts_spec

Instead of returning an NetApp::Facts object in the `facs` helper
function we can just return the facts hash. This not only reduces code
duplication but can also help when having multiple expectations about
facts. E.g. when writing a spec test like

    facts["foo"].should == "bar"
    facts.should have_key("awesome")

The retrieve method is only called once cause `let` only evaluates the
passed block once for each test.
Handle the fact that net-ifconfig-get is not available

In case the API call net-ifconfig-get is not available the API call
fails. As a result we should not expect to can access any child
elements.

Check the result of the net-ifconfig-get api call and skip the interface
facts if it the result is "failed".

The unit tests are suboptimal here because we don't test the positive
branch in case the api calls succeed. Unfortunately I don't have a
system available where the API calls works so I don't now how such an
answer would look like.