vhnd_availability() changed to fluff

http://manage.dimagi.com/default.asp?129242#727747
`get_dates` returns a list which is not going to be useful in as a group_by value. I understand that this gets overridden by the group_by values returned from the calculator but it should still be a valid value. Considering that the default value in the calculator is `[case.domain, '']` I would suggest using the same value here. This would also mean that you don't need to override the value in the calculator.