DRY-up overriding shortcode with sender

This avoids duplicating the logic when we add a new provider.

app/clients/sms/__init__.py

@@ -23,6 +23,7 @@ class SmsClient(Client):
     def init_app(self, current_app, statsd_client):
         self.current_app = current_app
         self.statsd_client = statsd_client
+        self.from_number = self.current_app.config.get('FROM_NUMBER')
 
     def record_outcome(self, success):
         log_message = "Provider request for {} {}".format(
@@ -39,6 +40,7 @@ class SmsClient(Client):
 
     def send_sms(self, to, content, reference, international, sender=None):
         start_time = monotonic()
+        sender = self.from_number if sender is None else sender
 
         try:
             response = self.try_send_sms(to, content, reference, international, sender)