Numerous model-related updates and supporing changes, including:

- Add variables field on Host/Group models and remove separate VariableData model.
- Add data migrations for existing variable data.
- Update views, serializers and tests to keep roughly the same API interface for variable data.
- Add has_active_failures properties on Group/Host models to provide indication of last job status.
- Add job_tags field on JobTemplate/Job models to specify tags to ansible-playbook.
- Add host_config_key field to JobTemplate model for use by empheral hosts.
- Add job_args, job_cwd and job_env fields to Job model to capture more info from running the job.
- Add failed flag on JobHostSummary model.
- Add play/task fields on JobEvent model to capture new context variables from callback.
- Add parent field on JobEvent model to capture hierarchy of job events.
- Add hosts field on JobEvent model to capture all hosts associated with the event (especially useful for parent events in the hierarchy).
- Removed existing Tag model, replace with django-taggit instead.
- Removed existing AuditLog model, replacement TBD.

requirements.txt

@@ -7,6 +7,7 @@ django-devserver
 django-extensions
 django-filter
 django-jsonfield
+django-taggit
 djangorestframework
 ipython
 markdown