fix celery task reaper

* celery workers have internal queue names that are named after the
system hostname. This may differ from what tower knows the host by,
Instance.hostname
This adds a mapping so we can convert internal celery names to Instance
names for purposes of reaping jobs.

awx/main/models/ha.py

@@ -45,6 +45,11 @@ class Instance(BaseModel):
 
     uuid = models.CharField(max_length=40)
     hostname = models.CharField(max_length=250, unique=True)
+    system_hostname = models.CharField(
+        max_length=255,
+        db_index=True,
+        help_text="Machine hostname",
+    )
     created = models.DateTimeField(auto_now_add=True)
     modified = models.DateTimeField(auto_now=True)
     last_isolated_check = models.DateTimeField(
@@ -109,6 +114,10 @@ class Instance(BaseModel):
     def jobs_total(self):
         return UnifiedJob.objects.filter(execution_node=self.hostname).count()
 
+    @property
+    def celery_system_hostname(self):
+        return 'celery@{}'.format(self.system_hostname)
+
     def is_lost(self, ref_time=None, isolated=False):
         if ref_time is None:
             ref_time = now()