Add route secret key header to the API requests

Currently requests to the API made from the admin app are going from
PaaS admin app to the nginx router ELB, which then routes them back
to the api app on PaaS.

This makes sense for external requests, but for requests made from
the admin app we could skip nginx and go directly to the api PaaS
host, which should reduce load on the nginx instances and
potentially reduce latency of the api requests.

API apps on PaaS are checking the X-Custom-Forwarder header (which
is set by nginx on proxy_pass requests) to only allow requests going
through the proxy.

This adds the custom header to the API client requests, so that they
can pass that header check without going through nginx.

app/notify_client/api_key_api_client.py

@@ -10,11 +10,6 @@ class ApiKeyApiClient(NotifyAdminAPIClient):
     def __init__(self):
         super().__init__("a" * 73, "b")
 
-    def init_app(self, app):
-        self.base_url = app.config['API_HOST_NAME']
-        self.service_id = app.config['ADMIN_CLIENT_USER_NAME']
-        self.api_key = app.config['ADMIN_CLIENT_SECRET']
-
     def get_api_keys(self, service_id, key_id=None):
         if key_id:
             return self.get(url='/service/{}/api-keys/{}'.format(service_id, key_id))