Apply capacity algorithm changes

* This also adds fields to the instance view for tracking cpu and
  memory usage as well as information on what the capacity ranges are
* Also adds a flag for enabling/disabling instances which removes them
  from all queues and has them stop processing new work
* The capacity is now based almost exclusively on some value relative
  to forks
* capacity_adjustment allows you to commit an instance to a certain
  amount of forks, cpu focused or memory focused
* Each job run adds a single fork overhead (that's the reasoning
  behind the +1)

awx/main/managers.py

@@ -2,12 +2,9 @@
 # All Rights Reserved.
 
 import sys
-from datetime import timedelta
 import logging
 
 from django.db import models
-from django.utils.timezone import now
-from django.db.models import Sum
 from django.conf import settings
 
 from awx.main.utils.filters import SmartFilter