introduce a new global flag for denoating K8S-based deployments

- In K8S-based installs, only container groups are intended to be used
  for playbook execution (JTs, adhoc, inventory updates), so in this
  scenario, other job types have a task impact of zero.
- In K8S-based installs, traditional instances have *zero* capacity
  (because they're only members of the control plane where services
  - http/s, local control plane execution - run)
- This commit also includes some changes that allow for the task manager
  to launch tasks with task_impact=0 on instances that have capacity=0
  (previously, an instance with zero capacity would never be selected
  as the "execution node"

This means that when IS_K8S=True, any Job Template associated with an
Instance Group will never actually go from pending -> running (because
there's no capacity - all playbooks must run through Container Groups).
For an improved ux, our intention is to introduce logic into the
operator install process such that the *default* group that's created at
install time is a *Container Group* that's configured to point at the
K8S cluster where awx itself is deployed.

awx/main/scheduler/task_manager.py

@@ -70,7 +70,7 @@ class TaskManager():
         '''
         Init AFTER we know this instance of the task manager will run because the lock is acquired.
         '''
-        instances = Instance.objects.filter(~Q(hostname=None), capacity__gt=0, enabled=True)
+        instances = Instance.objects.filter(~Q(hostname=None), enabled=True)
         self.real_instances = {i.hostname: i for i in instances}
 
         instances_partial = [SimpleNamespace(obj=instance,
@@ -86,7 +86,7 @@ class TaskManager():
                                                   capacity_total=rampart_group.capacity,
                                                   consumed_capacity=0,
                                                   instances=[])
-            for instance in rampart_group.instances.filter(capacity__gt=0, enabled=True).order_by('hostname'):
+            for instance in rampart_group.instances.filter(enabled=True).order_by('hostname'):
                 if instance.hostname in instances_by_hostname:
                     self.graph[rampart_group.name]['instances'].append(instances_by_hostname[instance.hostname])
 
@@ -528,7 +528,10 @@ class TaskManager():
                     break
 
                 remaining_capacity = self.get_remaining_capacity(rampart_group.name)
-                if not rampart_group.is_container_group and self.get_remaining_capacity(rampart_group.name) <= 0:
+                if (
+                        task.task_impact > 0 and  # project updates have a cost of zero
+                        not rampart_group.is_container_group and
+                        self.get_remaining_capacity(rampart_group.name) <= 0):
                     logger.debug("Skipping group {}, remaining_capacity {} <= 0".format(
                                  rampart_group.name, remaining_capacity))
                     continue