Use global capacity algorithm in serializer

The task manager was doing work to compute currently consumed
capacity, this is moved into the manager and applied in the
same form to the instance group list.

awx/main/scheduler/tasks.py

@@ -3,7 +3,7 @@
 import logging
 
 # Celery
-from celery import task
+from celery import Task, task
 
 # AWX
 from awx.main.scheduler import TaskManager
@@ -15,6 +15,12 @@ logger = logging.getLogger('awx.main.scheduler')
 # updated model, the call to schedule() may get stale data.
 
 
+class LogErrorsTask(Task):
+    def on_failure(self, exc, task_id, args, kwargs, einfo):
+        logger.exception('Task {} encountered exception.'.format(self.name), exc_info=exc)
+        super(LogErrorsTask, self).on_failure(exc, task_id, args, kwargs, einfo)
+
+
 @task
 def run_job_launch(job_id):
     TaskManager().schedule()
@@ -25,7 +31,7 @@ def run_job_complete(job_id):
     TaskManager().schedule()
 
 
-@task
+@task(base=LogErrorsTask)
 def run_task_manager():
     logger.debug("Running Tower task manager.")
     TaskManager().schedule()