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
Merge branch 'dev' into stage
Used the rtucker approach

Using a choice field and overriding __init__
rtucker's original code plus some minor fixes.
Added cost center model
Added cost center field to forms and templates
Merge pull request #2 from rtucker-mozilla/dev

Dev
Merge branch 'dev' into stage