Fix DEFAULTS method in src/rolekit/server/dbusrole

- Fixed the name of the property method
- Use try to not trace back for unknown properties
improved testrole

- Added missing imports for dbus type conversion and possible method calls
- Dropped "backup_paths" setting from _DEFAULS
- Added own setting "myownsetting" altogether with an extended get_dbus_property method
- Added not covered "failonthis" setting to show error message in the log with get_dbus_property calls
Add placeholders for Domain Controller and Database roles