Set dialog checkbox field value to "t" or "f" when clicked

https://bugzilla.redhat.com/show_bug.cgi?id=1132014
Checkbox dialog field needs to be checked if field is marked as required

Refactored validate method to breakout required_value_error? method to allow sub-class override.
Created validate_error_message method for sub-class override as well.

https://bugzilla.redhat.com/show_bug.cgi?id=1132014
Merge pull request #445 from dclarizio/set_correct_checkbox_field_values

Set correct service dialog checkbox field values and validate properly if Required is set