Kafka alert generator

Addresses #388 (closed), and depends on !851 (merged).

Feb 11 edit: First attempt at adding the code to publish to a kafka topic is done, needs testing. Alerts currently set to publish to StandinKafkaTopic

Aug 10 edit: Ready to be reviewed. Need new release of hop-client for a PR needed for GCN support (merged) and need to update URL of avro schema files once userguide!124 (merged) is merged.

Sep 21 edit: Peeled a lot of these changes off into other MRs, specifically !914 (merged), !915 (merged), and !917 (merged). This MR now primarily deals with the work needed to plug the kafka alerts into the orchestrator.

To-Do list:

• Test sending avro packet to kafka producer
• Decide where to store avro schema files for deployed instances to read
• Write unit tests to...
  • Validate that avro packet constructed by voevent is correct
  • Check that avro packet is sent to kafka producer

gwcelery/conf/playground.py

 """Application configuration for ``gracedb-playground.ligo.org``."""
 
+from base64 import b64encode
+
+from hop.models import JSONBlob, AvroBlob
+
 from . import *  # noqa: F401, F403
 
 sentry_environment = 'playground'
@@ -18,11 +22,13 @@ mock_events_simulate_multiple_uploads = True
 """If True, then upload each mock event several times in rapid succession with
 random jitter in order to simulate multiple pipeline uploads."""
 
-kafka_urls = {
-    'scimma': 'kafka://kafka.scimma.org/igwn.gwalert-playground',
-    'gcn': 'kafka://kafka.test.gcn.nasa.gov/igwn.gwalert'
+kafka_alert_config = {
+    'scimma': {'url': 'kafka://kafka.scimma.org/igwn.gwalert-playground',
+               'serialization_model': AvroBlob, 'skymap_encoder': lambda _: _},
+    'gcn': {'url': 'kafka://kafka.test.gcn.nasa.gov/igwn.gwalert',
+            'serialization_model': JSONBlob, 'skymap_encoder': b64encode}
 }
-"""Kafka broker URLs"""
+"""Kafka broker configuration details"""
 
 voevent_broadcaster_address = ':5341'
 """The VOEvent broker will bind to this address to send GCNs.