PXE objects and tests

* Added ScriptBox element
 * ScriptBox() class added for customization template page
* Added some extra logging for flash messages
* Added page_controls_exist() method
* Simple docs fix
* Added entire suite of page models for PXE
 * PXEServer()
 * CustomizationTemplate()
 * SystemImageType()
* Added tests for PXE
 * Added PXE Server CRUD tests
 * Added Customization Template CRUD tests
 * Added System Image Type CRUD tests
 * Added RHEL template file