only include cases owned by group in export

http://manage.dimagi.com/default.asp?144538

@czue since you were the last to touch this code: https://github.com/dimagi/commcare-hq/commit/e7526f1107ef7ec6ba65c3dbd04d95d4ea89d56b
do you know how the `groups` param is used? seems odd that it wouldn't be doing the same thing. might make sense to rename one of them, e.g. call this `filter_group` or something