Add validation when setting peers

- cannot peer to self
- cannot peer to instance that is already peered to self

Other changes:
- ReceptorAddress protocol field restricted to choices: tcp, ws, wss
- fix awx-manage list_instances when instance.last_seen is None
- InstanceLink make source and target unique together
- Add help text to the ReceptorAddress fields

Signed-off-by: Seth Foster <fosterbseth@gmail.com>

awx/main/management/commands/list_instances.py

@@ -55,7 +55,7 @@ class Command(BaseCommand):
 
                 capacity = f' capacity={x.capacity}' if x.node_type != 'hop' else ''
                 version = f" version={x.version or '?'}" if x.node_type != 'hop' else ''
-                heartbeat = f' heartbeat="{x.last_seen:%Y-%m-%d %H:%M:%S}"' if x.last_seen and x.capacity or x.node_type == 'hop' else ''
+                heartbeat = f' heartbeat="{x.last_seen:%Y-%m-%d %H:%M:%S}"' if x.last_seen else ''
                 print(f'\t{color}{x.hostname}{capacity} node_type={x.node_type}{version}{heartbeat}{end_color}')
 
             print()