SCM inventory cancel propagation

For manually initiated inventory updates, also cancel the source
project of "sync" type, like jobs do

For automatic inventory updates spawned from source project update,
of launch type "scm", handle contigency cases

awx/main/models/projects.py

@@ -501,6 +501,13 @@ class ProjectUpdate(UnifiedJob, ProjectOptions, JobNotificationMixin):
                         update_fields.append('scm_delete_on_next_update')
             parent_instance.save(update_fields=update_fields)
 
+    def cancel(self, job_explanation=None):
+        res = super(ProjectUpdate, self).cancel(job_explanation=job_explanation)
+        if res and self.launch_type != 'sync':
+            for inv_src in self.scm_inventory_updates.filter(status='running'):
+                inv_src.cancel(job_explanation='Source project update `{}` was canceled.'.format(self.name))
+        return res
+
     '''
     JobNotificationMixin
     '''