Switch DNS aggregator to PostgreSQL

MariaDB+Galera was terribly unstable, with the cluster failing to
start or dying randomly, and generally seemed incredibly unsuitable
for an HA solution. This commit switches the DNS aggregator SQL
backend to PostgreSQL, implemented via Patroni HA.

It also manages the Patroni state, forcing the primary instance to
follow the PVC coordinator, such that the active DNS Aggregator
instance is always able to communicate read+write with the local
system.

This required some logic changes to how the DNS Aggregator worked,
specifically ensuring that database changes aren't attempted while
the instance isn't actively running - to be honest this was a bug
anyways that had just never been noticed.

Closes #34

node-daemon/pvcd/NodeInstance.py

@@ -269,7 +269,26 @@ class NodeInstance(object):
             for network in self.d_network:
                 self.d_network[network].createGateways()
                 self.d_network[network].startDHCPServer()
-            time.sleep(0.5)
+            time.sleep(1)
+            # Force Patroni to switch to the local instance
+            self.logger.out('Setting Patroni leader to this node', state='i')
+            retcode, stdout, stderr = common.run_os_command(
+                """
+                patronictl
+                    -c /etc/patroni/config.yml
+                    -d zookeeper://localhost:2181
+                    switchover
+                    --candidate {}
+                    --force
+                    pvcdns
+                """.format(self.name)
+            )
+            if stdout:
+                self.logger.out('Successfully switched Patroni leader\n{}'.format(stdout), state='o')
+            else:
+                self.logger.out('Failed to switch Patroni leader\n{}'.format(stderr), state='e')
+            time.sleep(1)
+            # Start the DNS aggregator instance
             self.dns_aggregator.start_aggregator()
 
     def createFloatingAddresses(self):