reregister node when they come back online

* Nodes are marked offline, then deleted; given enough time. Nodes can
come back for various reasions (i.e. netsplit). When they come back,
have them recreate the node Instance if AWX_AUTO_DEPROVISION_INSTANCES
is True. Otherwise, do nothing. The do nothing case will show up in the
logs as celery job tracebacks as they fail to be self aware.

awx/main/managers.py

@@ -8,6 +8,7 @@ from django.db import models
 from django.conf import settings
 
 from awx.main.utils.filters import SmartFilter
+from awx.main.utils.pglock import advisory_lock
 
 ___all__ = ['HostManager', 'InstanceManager', 'InstanceGroupManager']
 
@@ -86,6 +87,20 @@ class InstanceManager(models.Manager):
             return node[0]
         raise RuntimeError("No instance found with the current cluster host id")
 
+    def register(self, uuid=settings.SYSTEM_UUID, hostname=settings.CLUSTER_HOST_ID):
+        with advisory_lock('instance_registration_%s' % hostname):
+            instance = self.filter(hostname=hostname)
+            if instance.exists():
+                return (False, instance[0])
+            instance = self.create(uuid=uuid, hostname=hostname)
+        return (True, instance)
+
+    def get_or_register(self):
+        if settings.AWX_AUTO_DEPROVISION_INSTANCES:
+            return self.register()
+        else:
+            return (False, self.me())
+
     def active_count(self):
         """Return count of active Tower nodes for licensing."""
         return self.all().count()