Kafka producer bootstep

Addresses #385 (closed).

This is a WIP. In particular, I'm not sure if the send() method should live inside the Producer, and what other arguments it should have.

The Kafka bootstrap configuration is also currently a placeholder.

To-dos:

• Add new packages as dependencies (currently suffering from the htcondor dependency issue on Macs)
• Choose between python-confluent-kafka, adc-streaming, and hop-client (or some combination) [updated Feb 7, 2022]
• Add correct host to configuration

gwcelery/conf/__init__.py

@@ -80,6 +80,12 @@ then completely disable the GCN listener."""
 email_host = 'imap.gmail.com'
 """IMAP hostname to receive the GCN e-mail notice formats."""
 
+kafka_urls = {
+    'scimma': 'kafka://kafka.scimma.org/igwn.gwalert-playground',
+    'gcn': 'kafka://kafka.test.gcn.nasa.gov/igwn.gwalert'
+}
+"""Kafka broker URLs"""
+
 superevent_d_t_start = {'gstlal': 1.0,
                         'spiir': 1.0,
                         'pycbc': 1.0,