foreman integration ui fixes

- workaround for problem with blinking multi select chosens
- possibility to deselect values in single select chosens
- refactored function for getting values from multiple elements at once
subnets - required attributes in model
architectures - fixed removing all OSs on update

Ajax put method can't send empty arrays. All empty arrays are automatically
removed from data in the request. Therefore we have to check such situation
and init fields the Simple CRUD Controller requires.
Merge pull request #1405 from tstrachota/ui_fixes

Various Foreman related UI fixes