hlr: pyhss: add ussd configuration
Configure the same strings for returning MSISDN and IMSI as in OsmoHLR,
because this is what the tests expect.
Related: https://github.com/nickvsnetworking/pyhss/pull/302
Change-Id: I7d2f62e6c034e3fd3c5dce048ccb297f4dea7825
