gbproxy: Switch to IP-SNS on the BSS side
When testing gbproxy with an IP BSS, we want to use IP-SNS as that
is the more relaistic use case in practice.
This un-breaks the dockerized tests since I90bd101096979b170c38fa2a80abb80d296c4d2e
was merged in osmo-ttcn3-hacks.git
Change-Id: Id1324e019b7f148f8d33977c0089e996089e1e59
