* ns2: add TC_sns_sgsn_add_change_del similiar to the bss side
* ns2: add TC_sns_sgsn_del to test SNS DEL on the SGSN side
* ns2: mark TC_sns_bss_all_signalling_nsvcs_failed as broken
* ns2: add TC_sns_sgsn_add to test SNS ADD on the SGSN role
* ns2: add first sgsn test case TC_sns_sgsn_config_success
* ns2: introduce module parameter SnsRole