added service_center setting to gsm backend.
added service_center setting to gsm backend.

caktus/rapidsms,ken-muturi/rapidsms,lsgunth/rapidsms,rapidsms/rapidsms-core-dev,eHealthAfrica/rapidsms,caktus/rapidsms,rapidsms/rapidsms-core-dev,eHealthAfrica/rapidsms,ehealthafrica-ci/rapidsms,dimagi/rapidsms-core-dev,catalpainternational/rapidsms,lsgunth/rapidsms,peterayeni/rapidsms,catalpainternational/rapidsms,caktus/rapidsms,unicefuganda/edtrac,unicefuganda/edtrac,dimagi/rapidsms,peterayeni/rapidsms,ken-muturi/rapidsms,peterayeni/rapidsms,unicefuganda/edtrac,catalpainternational/rapidsms,ehealthafrica-ci/rapidsms,ehealthafrica-ci/rapidsms,ken-muturi/rapidsms,eHealthAfrica/rapidsms,dimagi/rapidsms-core-dev,catalpainternational/rapidsms,dimagi/rapidsms,lsgunth/rapidsms,lsgunth/rapidsms,peterayeni/rapidsms