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/models/projects.py

@@ -563,6 +563,8 @@ class ProjectUpdate(UnifiedJob, ProjectOptions, JobNotificationMixin, TaskManage
 
     @property
     def task_impact(self):
+        if settings.IS_K8S:
+            return 0
         return 0 if self.job_type == 'run' else 1
 
     @property