Fixes #6695: Adding two OS parameters with same name doesn't raise any error when values are different

Can anyone help me fix these two tests?
` FactoryGirl.create(:hostgroup, :with_parameter)` was supposed to use the trait `:with_parameter` which preformes
````ruby
trait :with_parameter do
      after_create do |hg,evaluator|
        FactoryGirl.create(:hostgroup_parameter, :hostgroup => hg)
      end
    end
but for some reason my validation causes a problem with creating parameters.
When i change it to manually create a parameter and add it to the group_parameters the test passes