Fix detecting if peers changed in serializer

Add a check_peers_changed() utility method
to determine if peers in attrs matches
the current instance peers.

Other changes:
- Set ip_address default to "", and do not
allow null.

awx/main/models/ha.py

@@ -105,8 +105,7 @@ class Instance(HasPolicyEditsMixin, BaseModel):
     hostname = models.CharField(max_length=250, unique=True)
     ip_address = models.CharField(
         blank=True,
-        null=True,
-        default=None,
+        default="",
         max_length=50,
     )
     # Auto-fields, implementation is different from BaseModel