Fixed issue with seemingly cached form results for RackFilterForm Bug #827420
Field type from ChoiceField to ModelChoiceField

ChoiceFields have their choices instantiated when the form class is created.
This is why it appeared to be 'cached'. ModelChoiceFields are a better choice
for the type of drop down being used.

I also cleaned up the view.
Merge branch 'rack_drop_down_fix2' into dev

Conflicts:
	systems/forms.py